Yes, they can get bigger. I too have a very small male leopard. My females are almost 30lbs, however my full grown male is only about 10lbs.
I'm pretty sure yours is going to grow a bit more. Just make sure you are keeping him according to our suggestions on the leopard tortoise care sheet to give him all the full advantages.
How does the vet know how old he is? Unless you know the hatch date there is no way of knowing how old they are.
That is very small for a tort of 8 years if hes really that old.