These are thick under my sons arms. Can't figure out how to treat them

First of all, be sure they are warts. You might need a doctor's exam to make sure.

Second, warts form because of a virus. Here's an eHow site that might help:

How to Remove Underarm Warts | eHow.com (http://www.ehow.com/how_5553059_remove-underarm-warts.html)