I am on Celexa/Lexapro and even 5 mg can make a huge difference. It is a lot of medicine. I also gained about 15 lbs once I started the SSRIs, it is pretty common - however - I do not remember food binging as an issue. It seemed to "just happen" - like my metabolism slowed.
I would take everyone's suggestions, as well as your own gut, and d/c the 5 mg and see where that takes him - and get him into to see a doctor for a full work up.
I have anxiety out the wazoo as well - and since most docs will not XR Xanax because of it's addition issues, I have had to just learn mega coping skills - some of which don't always work. It is hard. Therapy helps a lot.
Many hugs.
