Last I weighed him ( about 3 weeks ago) he was 575g, he never really lost weight, just never gained weight as quickly as any of my other bps. I have another bp that's 3 months younger than he is that was 732g the last I weighed him (also about 3 weeks ago, though he was a big boy to begin with.. I weigh monthly).
And thank you for the reassurance - when people jump to IBD it makes my heart palpitate even though I know that's not what it is
Weights vary even with bps the same age. Even one's fed the same. So its completely okay for one to be a couple hundred grams off from the other that's the same age. It doesn't mean they are underweight at all.