I'd stick with 2 rat pups until he's a bit bigger, just to avoid the risk of regurgitation.. If you think it looks too big, it might be too big.

Depends on the individual body of the snake though, weight doesn't help much. My 270g girl is still on one rat pup every 5 days, but she's just a heavy-bodied little girl, barely past being a "hatchling", and possibly a little overweight.