J.G.
My one year old daughter is a thumb sucker and started to get a rash from the constant exposure to moisture. I put a little lanolin breast ointment on her thumb before she goes to bed at night and it has helped tremendously. It should be safe even for your son who sucks his fingers constantly throughout the day since breastfeeding moms apply the ointment to their nipples and it doesn't have to be washed off before the baby nurses.