Ya Ritalin severely decreases my appetite which is why I take it every day. Is your son taking the Focalin every day? I think that the side effect goes away after taking it every day for at least 6 weeks.

It would be a good idea to take him off the med for the summer. I don't know how old he is but if he doesn't have any responsibilities then I'm sure that he would be happy to not take it unless his hyperactivity/impulsiveness is too distressing.

Be warned though that when he starts school again, he WILL lose some weight so make sure he eats at that time.