I have been a sufferer of what your son has for 25 years, its keratin pilaris, and will only worsen with age. Fortunately your son will never go through pregnancy (like I did) which will only make matters worse. Its hereditary and just because parents don't have this problem doesn't mean that their children won't. My 7 month old daughter is already showing signs of having it and it breaks my heart. There is NO CURE for this problem and it is completely harmless, however, I can tell you a few things that may make it better but the same things don't work for everyone.
1. Ya know those cheap exfoliating gloves you can buy at walgreens or wal-mart?? Purchase one
Children don't like to be scrubbed with it, but its the only thing that will help. You have to get as much of the dead skin and other things blocking the pore off of the skin. Wash with a VERY gentle soap. I use unscented Johnson's and Johnson's baby scrub. It has no dyes or harsh cleansers that cause irritation. You probably already use it for your little one so thats handy.
2. You may have to search for the hydrating cream that I use, but it does exist and works wonders. Its called Udderly Smooth. Its actually a cream used for cow udders (gasp! I know but don't worry). It has the consistancy of whipped cold cream and a pleasant smell. I use it on my daughter for her diaper rashed because desitin is so harsh. Its not all that expensive and worth every single penny! You will have to apply this 3 times a day at first, and once you see the bumps start to disappear, you can cut it back to 2 times, then 1. However, you may not be able to cut down on the use (I can't but I did with my daughter) it all depends on how bad the KP is.
3. Last but not least...Don't give up!! If the udderly smooth cream doesn't work keep trying other stuff. Don't stop exfoliating, just change the lotion and make sure to apply it a couple times daily. You will be able to tell after a week if that product is working or not. I noticed the Udderly Smooth working for me after just the second application on the very first day
The most helpful things for KP are Lanolin, Allantoin, and Dimethicone. Lanolin being the best. Don't go out and spend tons of money on retin-a creams because they are only a quick fix and once you stop using them, the KP will come back worse. The are terribly expensive (I have used creams ranging from $4-$100) and nothing worked until now.
I really hope this will work for your son. I'm trying really hard to get the word to as many people as I can that this cream really works. Like I said, I have suffered for 25 years and it is terribly embarrassing growing up with bumps all over your arms, face, and legs. If I can help just one other person then my own suffering and hopes to find something that would work to improve my skin weren't in vain.