I think that anxiety and depression can be treated early to avoid further damage, socialy, emotionaly and probably physicaly. Some medication may help the kid and if he doesn't have problems (who doesn't) with the side effects, it could hep him and you too.
Your son is not likely to experience side effects from this medication, and it is a reasonable choice to address anxiety. However, if the developmental pediatrician has not collaborated with a child psychiatrist about the treatment plan, I'd suggest this occur. Prozac is not the only available option, and the child psychiatrist is in a position to offer some special expertise in this regard.
my daughter was 3 when she was put on prozac for anxiety and behavioral problems. it was done as a kind of last resort, however i found that it did help quite a bit. she was more content playing by herself instead of constantly worrying about where i was and what i was doing. i didn't see any time of bad side effects at all.