C.L.
All kids develop differently and that may be perfectly normal for your son. Both of my boys are losing their baby teeth a little slower than other kids their age. However, I think it would be a good idea to at least call the dentist back and get a professional opinion. They can probably tell you if you should try again for x-rays. This doesn't sound like your son's situation, but my younger son lost a baby tooth, but the permanent tooth never came in. Apparently they can get stuck behind tissue sometimes. Our dentist had to make an incision in his gums to allow the permanent tooth to come in. Good luck.