L.A.
Yes its normal for children to chew on stuff, even up to 4 or 5. That seems odd, I know. My daughter still (4) will at times, put stuff in her mouth. Its not often, but she does. Since his dentist said they are great and he isn't teething, it probably is just the normal kid stuff. Not to worry. I know it isnt your most favorite habit of his Im sure......hang in there, he will stop.