Sounds fairly normal for his age, expect you breast fed him and 'bosoms " are very comfortable and pleasant arent they, hes a guy and even if you are his Mom he likes them, I doubt if it needs too much talk about them specifically but a chat about humans bodies may be a good thing, and maybe a slightly higher shirt when you are working with him, I knew a mom once who even after she stopped feeding her son would run up in public and fling himself at her bosoms sometim,es pulling her shirt up just to be able to get a cuddle that way. No one thought it was a bad thing we know it was Mom Love.she had fed her child till he was abou 2-3 years old , seems to be a guy thing ,