I agree with mjemom. Children that age are all senses, so to speak. They still like to touch, taste, and smell everything, it seems – or at least my children were that way. My 10 yo dd would constantly eat asian beetles (UGH!) during this stage. The dr. told me what I just wrote above, and said as long as it wasn’t poisonous, it was fine – she was just getting some extra protein.
I know it’s terribly annoying to you, but if your dc knows that this bothers you, it may entice him to continue to do so simply to get a rise from you as well – my nephew is like this. On the other hand, you know your child best. If there are other indicators besides the sniffing, licking, etc that there may be another underlying issue, then you should follow your instincts on this. God has given this child to you, and you know him best.
~jacqleene