As a teacher, I always knew how my student's drugs were effecting them. I'd schedule a conference with his teacher and ask if she's noticed any changes at school. Teacher's are usually very open if you make them feel that you're just asking for an education opinion, not a medical one.
I'd contact NAMI (National Alliance for Mental Illness), a group founded by the families of people with mental health issues. I don't like the name, but they do good work and can help make a good referral for a Psychatrist or Psychologist. Please consider the fact that your ex-wife may be overwhelmed and desparate, as many parents of bipolar children are. I take a lot of meds myself, and they work really well until they need to be adjusted, and then it's really hard.
Also, I'd try really hard to work with your ex to get the best combo of drugs. Like it or not, she needs to be part of the solution.
I'm sorry to say but the long term effects can be devastating. Always talk to a dr about weening off of any med. It sounds like your ex doesn't want to really deal with the boy on a regular basis so she drugs him to keep him plyable. I had a dr do that to me. You should go see your son's shrink and tell him of your concerns and see what he says. If you get no help there, try the medical psychiatric board where you are. They have to look into suspected overdosing, especially of children. Good luck. Shane