I have one eastern male who is about three years old, and up until very recently he was small enough that I thought something might be off, but now he's the same size as the other eastern male he lives with; maybe a bit bigger. I have another eastern male who is an absolute giant; almost twice the size of my others, and easily mistaken for a female.

I do have one male blackneck that was nearly ftt who is definitely stunted in some way. He's probably about a year and a half, and is still extremely small for his age. Has mild neuro issues, but those have improved with age and he eats very regularly.

Basically, it can be a health problem, or it can just be how the snake is growing. Odds are he's fine and is just a 'late bloomer'. A picture with something like a ruler as a size comparison might help people tell you if he's behind enough in growth to really worry about though.