Hi there, Thank you for choosing healthcaremagic. I will try to address your concern. Your 21 month old as you mentioned has had a sore on the tongue as well as is irritable and has decreased apetite. He is probably suffering from a viral infection like hand, foot and mouth disease or viral gingivostomatitis causing him to unable to eat properly and the sore in his mouth. These viral infections can also cause sores or ulcers in other parts of the mouth as well and very frequently hand, foot and mouth disease can cause blisters on the hands,buttocks and feet. These are self limiting infections that take their own time to resolve. Till then, you can give him acetaminophen or typenol for pain from the sores in his mouth or fever. Continue to encourage him to drink oral rehydration solution or pedialyte. Also, I would suggest you visit his pediatrician for a complete evaluation so his doctor can examine his mouth and look for other rashes and confirm the diagnosis and rule out other things and treat accordingly. Hope this has helped you. Regards, Dr. Sumeet.