Without a photo it is hard to tell, but given the large areas, he may have a common condition called keratosis pilaris. With that, the follicles get blocked up and dead skin cells as well as a tiny hair accumulates and creates a blockage. It may be that he will outgrow this. If this is what he has, I recommend keeping the skin moisturized, and you can get some over the counter lotions with alpha hydroxy/glycolic or lactic acid. You can gently scrub the area, such as with a buff puff. I also recommend my patients supplement with some omega 3 fish oils. They tend to come in rather large capsules, so look for a liquid formulation, unless he is good at swallowing pills. Make sure he has a diet that is healthy, without a lot of processed and refined foods.