Brief Answer:
No need for concern
Detailed Answer:
Namasthe
Dear Ms XXXXXXX
As you have mentioned, he used to speak before and say amma,baba, attha that means he has achieved the milestones of language . It will not regress back, so there is absolutely no cause for concern.
He pays attention to whatever you say, he enjoys your company that is a very good sign indeed. He has undergone Surgery just 2 months ago, and he is in cast so he will require some adjustment to his new environment. ( Imagine even we being adults, if we undergo surgery and are in cast all the day and not able to walk, even we would be behaving something like this). This is the age when babies are very active and moving about, but due to his cast the movements are restricted. Do not worry, he will start speaking again. All that is needed is continuous efforts from your side. Keep speaking to him, see how he reacts, play music especially bed time lorri.
Also i would like to rule out one more thing at this time, In my clinical practice i always rule out Tongue tie as the cause(is he able to get the tongue out of mouth?, sometimes its stuck in the floor of the mouth, so may face some issue).
Do not worry, your child can hear, his other milestones are normal, he had already vocalized so there seems to be no cause for concern.
For your reference i will enumerate the speech milestones
Birth to 3 months
Crying is your baby's first form of communication. Your newborn makes quiet, cooing sounds when he's happy or content. These are typically single vowel sounds, such as "ooh" and "ah" with his tongue and lips.
2 to 3 months Your baby alters his cry according to the situation. As you get to know your baby, you may be able to distinguish a hunger cry from the cry he makes when he's tired, has
colic or is uncomfortable.
3 to 4 months Your baby cries less frequently but will still cry when he's uncomfortable or annoyed. He utters mostly vowels, but cooing and gurgling is now a little more sophisticated, with more varied sounds added. Consonant sounds will begin to mix with vowels, such as "mama", "baba" or "yaya".
5 to 6 months Your baby practises making his voice rise and fall (intonation), often in response to your baby talk and facial expressions. He'll be able to make sounds to get your attention, and will babble to himself and laugh when he plays.
Ask for help if ... If your child isn't making vocal sounds or eye contact with you by the time he's six months old, talk to your doctor.
7 to 12 months Your baby is babbling with greater diversity by now, making new sound combinations and intonations. He tries to copy your speech by putting consonants and vowels together, such as "bah-bah-bah" or "no-no".
He'll have pretend conversations with you, taking turns to talk. He'll recognise the names of familiar objects and toys, such as "car", "ball", "phone" and "teddy". By 10 months most babies will respond to their name.
Toddler
12 months The all-important first words will have been spoken by now, and were probably "mummum" or "dada". Your toddler may know up to five words well enough to use them, but not all may be clear to understand.
15 months Your toddler raises his voice at the end of a question and makes hand gestures to complement his speech, such as pointing and waving. He may even be able to say up to six simple words.
16 months Talking rather than just babbling out loud has started, even if it isn't all that clear. He calls you to get your attention and nods and shakes his head for yes and no. Common consonant sounds such as t, d, n, w and h become part of his speech.
18 months Your toddler may have a vocabulary of between six and 20 words, including nouns "mama", verbs such as "eat", and adjectives such as "cold".
18 to 24 months Your toddler may have started to put two and three-word phrases together to form simple sentences, such as "Daddy go", "Bye bye Dadi", "bath time" or "potty time".
By 19 months :about half of all children will be doing this. He'll sometimes
ask questions about the names of people and objects around him.
24 months Your child may know as many as 50 words by now, including some descriptive language. He'll use short sentences consisting of two or more words with lots of pauses, such as "want milk", "Baby ... fall down!" and "Me go... park?".
2 to 3 years Your child knows up to 300 words by now, including lots of verbs, and will be able to carry on a simple conversation about something in his immediate environment.
He'll be asking simple questions, such as "What?", "Where?", "Why" and "Who?" a lot. Phrases expand from three-word sentences to five. He may stutter or stammer while he's thinking what to say, but this is normal.
Ask for help if ... If your two or three-year-old echoes your questions rather than answering them, or has been
stuttering for more than six months, speak to doctor.
Preschooler
3 to 4 years Your child will make words past tense by adding a "d" sound to verbs, such as "runned", and can talk about things in the future as well. He'll make plurals by adding an "s" sound to nouns, such as "mans" and can use pronouns (I, she, we) correctly.
He can talk and play at the same time, knowing between 800 words and 1,000 words. He can say sentences of four to six words and repeat his favourite words such as "why," "what," "how" and "who" often.
You can understand him most of the time, and he can describe what happened if you were out of the room. By five years speech is generally grammatically correct.
Remember this is just a rough guide and there is no hard rule. Every child is unique and has their own developmental milestones. ( Its like if you plant 100 tress simultaneously in the same land they will grow at different rate, same thing is true with our kids). Being a father myself I understand your concern and apprehensions. Do not worry there is no cause for concern.
Do write to me if you need any further assistance.
Take Care Madam
Regards
Deepak