My boyfriend get sick at least 2 or 3 times a month, is there any vitamins he could take to help his sistem to be stronger?

MANY proven to help!: Research shows that vitamins a, b complex (esp. B6, B12 & folic acid) c, d, e & k all enhance immune function. Bioflavonoids & minerals like zinc are also important for immune function. Many herbs like astragalus, ashwagandha, andrographis & medicinal mushrooms like coriolus help. Proboost thymus hormone works well. I recommend the book the road to immunity by kenneth bock md for detailed info.

A multivitamin: In general, it is recommended that one take a multivitamin daily, unless one is on a restricted diet (i.e. Vegan) or has an intestinal problem that would result in decreased absorption of some vitamins and minerals.