Those medications are notorious for severe weight gain. How is his mood and affect? Does he enjoy any forms of exercise? Or going outside, walking near nature, hikes, water, etc? I would help him with that.
The lap band is not a good surgery, in general. Many weight loss surgeons refuse to do it anymore due to complications. It can cause erosions, severe acid reflux, can even wear its way into the stomach and require major surgery to repair. I would suggest the Gastric Sleeve operation over the Gastric Band. Plus, your son would have to go frequently to get 'fills' on the band...a huge pain in the rear. Too tight, and he will not be able to swallow anything down. Too loose, and he won't have good restriction and will lose little to no weight. I would suggest the Gastric Sleeve.
But your son has to be stable. I am not sure if insurance will cover the surgery for someone if they are not stable and out of the mental ward for a long time. Also, DO NOT go to Mexico for cheapie weight loss surgery (though you will have a ton of scammers posing as 'success stories' online urging you to go to specific doctors in Tijuana, etc) There are TONS of patients who have died trying to pay for the surgery themselves at a quarter to half the price it costs in the US. If you want to self-pay for the sleeve, the bet rate I have seen is a doctor in Boca Raton who does Gastric Sleeve for the jaw dropping price of $10,500 (it's usually about $20-30K in my busy metropolitan area).
Best decision I've ever made -130pds 4 years!!!