My DCs both lost their teeth late, despite being big for their ages - at 9, DD could have been taken for a 13-year-old, apart from the gap-toothed smile. She is now 14 and I think lost the last one or two last year.
There is a wide age range for losing teeth, and 9 is later than average but still normal. If you are at all worried, ask your dentist.
I lost my first at about 6 but it was very slow! Only about 1 or 2 a year. Last 4 had to be pulled put at 14 as they were refusing to budge... I was NOT happy as the adult teeth didn't come through fully until I was 17 - I had a gappy smile for 3 years!!! Don't worry about it, strong teeth are a good thing! I'm an adult and have no fillings and straight teeth now so I'm rather happy I was a late developer (in the tooth regard)
I think it all depends on how quickly their baby tooth came. My dd got her first tooth aged 15m and was still erupting her molars aged 2, so I'd expect them to fall out later aswell. She's 9 now and has only lost 6 teeth aswell.
Ask your dentist to take an x-ray to check things over. My ds lost teeth and then spent ages with gaps whilst the new ones came through. My df's son lost his much later and his gaps were filled almost immediately.
For what it's worth I still have milk teeth and there aren't any adult teeth to come through.
My ds is 11 and has only lost 2 of them were lost last week. I asked his dentist as I had noticed all of his friends seemed to have all their big teeth but my dentist said the later the better to get big teeth!
I've a 7yo who has only lost the centre two lower teeth. Most of his friends have lost many more and have gobs full of big teeth. I asked the dentist and she said its nothing to worry about. They lose and grow teeth at very different rates.
I had 4 milk teeth left at 11yo, 1st two sets of molars next to canines top and bottom, and had them all pulled. 4 under gas, 4 with injection. This was wayyyyyyy back in the early 90s, happy to see they are just leaving them as is now. The gas was scary - they were panicking that I wasn't coming around and the 4 with injection made me faint twice, i was so scared.
Dentist said the roots of my baby teeth weren't dissolving, even though my big teeth were there - but not pushing the babies out. It was stalemate!
In other news, just-5Yo DS has lost two bottom middle teeth and has big ones coming in already! I am brushing them like a crazy person
I think it is within the normal range, my DB was slower losing his teeth than me and hadn't lost all of his by the end of primary school, and I had a school friend who hadn't lost any by year six. I felt bad for her as she had missed out on many a payment from the tooth fairy! I wouldn't be worried, he probably just has excellently strong milk teeth!