We just dealt with this with our daughter over the past couple months. She had two perm's that came in behind the baby teeth. When we first noticed it, I called our dentist. He told me to keep an eye on them, but he wasn't too concerned at that point in time. He said that if they came in to the point where we could clearly see them behind the baby teeth w/ her head level looking at her straight on then he would like to see her. His other advice was that if she complained of them causing any pain, (other than a lttle normal pain when she was eating) in either the babies or the perms to call his office. Generally as they come in they will dissolve the roots on the baby teeth, but it doesn't always happen. Sometimes if they do not have enough room they will deflect backwards and the roots will not dissolve enough to loosen the baby teeth for them to pull on their own.
This is what actually happened to my daughter. Her perms were showing behind her babies when we looked at her and at about that same time she started complaining that they were hurting her. I called the dentist for a consult. I took her in on a Thursday afternoon & the following Wednesday the dentist pulled her baby teeth b/c they were doing as I mentioned above. They were beginning to deflect backwards, which would have been BAD news for the future if we had let them go too long. When he pulled them he used local anesthetics, a topical gel & then a couple shots into the gums. After pullin them we saw that the roots on them had been dissolved on the back, but not at all on the front root, it was still longer than the tooth itself! Bc they were deflecting back, it would have never dissolved that front root allowing her to pull them. In the three weeks since she had them pulled her permanent teeth have already moved forward quite a bit!
So after my lengthy post I will suggest for you to keep a close eye on them! Don't mention the possible "pain" to your son, but listen to see if he starts complaining b/c you'll definitely want to get him in to see the dentist if he does.
On another note, it seems a lot of dentist don't seem to be concerned about it at all! My friends son, now 5 1/2 has had perms growing behind his babies since Sept. Her dentist refuses to do anything about it. His babies are not loosening, but since the dentist doesn't seem worried she's just letting it go. I would rather get it checked out then let it lead to problems in the future!