My 6 yr daughter just lost her 5th tooth... she started loosing the frount ones shortly after she started preschool at 4. So, I don't think your boy is to young to have a loose tooth.
Also, my kids started to see the dentist shortly after they had teeth... my littlest was around 6 mo old when he first started going. They keep telling me that there was no way he was 6 mo old w/ all the teeth he had & again at 12 mo I hear the samething... he had a full mouth of teeth. But he also loves to eat and you have to have teeth to do that - lol.
It's just time for you to start telling your boy about the tooth fairy... we have a special tooth plate that the tooth gets put on and in the morning the tooth is gone & a gold dollar is there. Remeber have fun with it!!!