In answer to your question about withdrawals from Strattera. My son (now 18 & no longer medicated) was placed on Strattera when it was still considered a "new" drug. He too was a thin boy...but mostly due to high energy levels and genetics. He did not lose weight while on the drug, but he did have a difference in appetite.
I would give him weekend and school break holidays from Strattera. I did this with the blessings of his pediatrician. I was told because Stattera is NOT a stimulant drug (no amphetamines) the reaction or withdrawal would not be so prevalent. The biggest difference I noticed were his energy levels would spike for the time he was not medicated, but he didn't seem to have much other effects.
I would talk to your sons doctor about your desire for "drug holidays" and see where he/she stands on the issue. Also with the weight loss your son is experiencing, have you considered giving him any kind of supplements? Not a weight gain supplement but just a nutritional one, like Ensure or something of the sort, just to make sure he is getting all the nutrients he needs.
Hope this helps...if even only a little
Angela