Bipolar meds and side-effects?

My 8 year old has been on Zyprexa for about a year now. He started at 5 mg then moved to 10 because on 5 he still wasn't sleeping at night (maybe 2-3 hrs max). They then bumped it to 15 mg because 10 wasn't working anymore. He went from 40-90 lbs in the year he has been on it. Blood test show no sign of any medical issues with the weight gain, glucose is normal which the Dr is happy about. They just moved him to 20 mg a few days ago. My concern is can Zyprex especially in larger doses lower immune systems or cause problems you may not see right away. My son has been feeling sick for awhile now especially the last year but recently he has gotten strep throat 3 times. Hand, foot and mouth disease. Ear infections. He has felt very weak in the legs causing him to not want to walk around the store and will fall very easily, bruising very easily. Anyone had this problem after taking bipolar meds for awhile. He is also on Concerta and Tegrital.