Hello
I really appreciate your concern and being your physician, I will try to help you out in best possible way
Looking at your description this could be suggestive of
injury or inflammation of during sex, STI or other infection and an allergic reaction
Chances of streptococci or staphylococci infection are less likely.
I would like further information to help you better
His age and relevant medical history
is he having any kind of discharge
In my opinion he should keep it clean and apply antiseptic ointment
If it gets worsen or doesn't improve than he should consult your doctor for physical check up and detailed evaluation
Maintain good hygiene
use condoms during sex until it heals completely
Hope this answers your question
if you have any more doubts please feel free to write back to me, I will be happy to help you
Best wishes