I am a 16 year-old girl also suffering from bipolar disorder, and now that I am on meds, my mania is no longer present and I have to deal with the horrible depression that my medicines cannot eliminate. Lithium is a very powerful, so powerful that it can make a person's life a lot harder than what it originally was. But it is also a good medication for bipolar disorder, and I encourage your son to stay on it if your doctor says so.
As I do believe that it is VERY hard to go to school when you are depressed, I don't believe he should be missing an entire semester of school. Is he on a special plan at school for his disorder? If not, I totally ache for him because that is the only way I can go to school. Our school system has a special plan for children with "disabilities" or people on meds that cannot focus, have a hard time concentrating, and just can't get their life on track. I get leeway on assignments when I am so depressed I can't do it, I get extra time and extra help on tests and assignments, and my teachers know all about my meds and my disorder, so they help SO much making me feel comfortable. This plan has saved my life, so I highly recommend talking to his school counselor at school. Even if your school does not have a plan, it is best having a teacher to lean on like his counselor when things get tough.
Talking on here is also a great way to vent. He probably won't want to, but let him know. This site has helped me to talk about it.
I take probiotics (acidophilus) and fish oil. These are thought to help depression significantly, and if not, it is VERY good for you so you might as well take them haha. Hope this helps!
God bless,
Allie