A visit to the dentist should put your mind to rest. The dentist can check to see if there's a physical reason why he's chewing with his front teeth more than the back ones. If it's just a habit, then he should grow out of it once his permanent teeth come through. If he's around 6 or 7 (and I presume he is because that's the age when the front baby teeth fall out), his first permanent molars should be in the process of erupting. Once they're fully erupted, he should be able to chew on them. And no, he will not damage his adult teeth (I presume you mean the as yet unerupted front permanent teeth underneath the baby teeth?) by continuing to chew on his front baby teeth.