If you are a full time working mother and can't afford the proper medication for your son, there has to be a problem somewhere (and I mean the system, NOT YOU!). If you are on your own, with such little medical support, there MUST be help at your local welfare office. I just can't understand how this happens, because it's not YOUR fault, you're working full time and STILL can't get medical needs covered.... geez. I'm so sorry this happens to moms in America. I'd pound on more doors and see if there is some financial support from somewhere to get the other medication and some serious counseling, mentoring, something, to completely change the track your son is on. Can the father offer more help and support? The father's employer?
All I can suggest is some DRASTIC action and the sooner the better. Kids fall into a rut and it becomes habit and the habits become very difficult to 'unlearn'. The confusion and feelings of helplessness, anger... it all becomes a permanent and constant negative self-talk in their heads and it will take a very big shift in their routine to change it.
Good luck. Keep some wits about you because you'd be surprised how they keep coming and don't seem to end when you think they're running out!