Thanks for writing back.
The joint pain and the pain in the legs do continue for a while after the person recovers from dengue
and chikungunya. These usually become ok by themselves and nothing as such needs to be done. If the pain is severe, your son can take Tablet crocin
1 tablet as and when required (upto 4 tablets daily).
This will become ok on its own. Nothing special needs to be done.
As far as the food is concerned, a balanced diet is required. Make sure he has plenty of iron ( found in legumes, dals, non vegetarian foods) and folic acid
( found in fruits and vegetables) to make sure he's able to make the platelets again.
Thats all thats required
Hope that helps