Weight Gain with Mirtazapine and Zoloft

Hi, I have a 12 year old son who is ADHD and Bipoloar he currently takes 50 mg of zoloft, 30 mg of Mirtazapine and 72 mg of Concerta per day. My concern is his weight, which has balloned to the point I am actually worried about heart stress. His Dr has continually repreminded me about his weight gain and I have told him his diet and excercise level have not changed, I was totally unaware that both zoloft and Mitazipine can cause massive weight gain and suppressed metabolism. I feel pretty stupid to be honest because I trusted these people to at least make me aware of these issues. Why would they not say that this might be a normal side effect AND reprimand me about his weight. I am beginning titration off of the Mirtazapine tonight and wonder how long I should wait before starting titration of the Zoloft. Can you do this at the same time or is it better to do it one at a time. I would like this crap out of his body as soon as possible. Thank You for any help you can give me.

Here's the problem, he is going to a pediatric psychiatrist the only one within a 200 mile radius. and obviously he is an idiot, I should have realized this when he put my son on seroquel about 3 years ago and within 4 days I had a 9 year old who told me he wanted to die and attempted to sit in the street in front of our house. I know its ideal to just go to another ped psy but in this case its not possible, so has anyone ever titrated themselfs off this. Seriously my poor baby walked for 10 minutes and had to stop to rest. Frankly I have had it with the medical community as a whole right now and I can't stand by and watch my child eat himself to death SO if you can help with this please post. Thank You