It really depends. We pulled my daughters first tooth when we saw the permanent tooth growing in behind it. It was already loose so I helped it along.
She lost the tooth right beside it a week or so later but it took a couple months for the permanent one to pop through. Her next 2 teeth probably took 2-4 weeks to come in.
She lost one the day after Christmas and it popped through a few days later.
I do know some adults who lost their baby teeth but there were no permanent teeth underneath. Also, it's possible for teeth to grow sideways--that is fairly common with widsom teeth but it can happen with other teeth.
At his next dentist visit they will probably take xrays and you can ask then. If he doesn't go to a dentist regularly, I suggest at least a baseline visit to check him out.
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