Thanks for your replies, everyone.
I know I should weigh Clyde, but he has always been a thin boy. The last two times when he had the spike filed down, the Vet nurse did not mention any problems with his molars. He was weighed at the vets the second time, and had actually PUT ON weight! He is still eating the same and as much as the other piggies, just takes a bit longer. Probably because I cut his veg up smaller. No, he has never been a bar-chewer. He does eat lots of hay and I cut long grass from the garden for all of them now and then. I will continue to keep a close eye on his teeth.