My son has small bumps on his back, chest, and inner thighs. What could it be (it is not chicken pox)?

Have to see. Too many possibilities. Needs to be seen to tell you. Go see your doctor.
Not sure. Of course can't give you a definitive diagnosis but there is a condition called molluscum contagiosum that is quite common in children in those locations. I would recommend checking some pics on www.Aad.Org and see if it looks similar.