It isn't really normal, no. Puppies begin losing their milk teeth and getting their adult teeth around 4.5 months, and the whole process is generally completed around 6-7 months.
But what does sometimes happen, is that the adult teeth come through but one or two milk teeth can remain. So the dog has duplicate teeth, if you like. That can cause the adult teeth to grow crooked, of course, so you would normally have the stubborn milk teeth pulled out if the adult teeth are coming through behind them. Anyway - that would be one possibility with your dog, and explains why you wouldn't see a vacant space. Another possibility is a chipped tooth. Couldn't hurt to have your vet take a look in his mouth next time you visit.