Osteoarthritis
 

My child is very interested in playing tennis. Is there any problem if he play tennis when he is free from pain?

It is very hard to give an answer to this, I'd suggest checking with the doctors.
I used to hit the tennis balls against a solid wall for fun.
A nice way to work on control & aim..
That way no collisions or falls.

Osteoarthritis is typically a "wear and tear" joint condition though it can have an inflammatory component as well. Usually it affects older people.

Does your son's pain reflect osteoarthritis? If so, as Jo*mar suggested, checking with the doctors is a good idea.

I have spoke with our family doctor, he said that there is no big problem in playing. If I restrict him from not playing he may feel bad.