If he's not skinny - BPs, even babies, should be more round than triangular - then he's okay. It's tough to say what exactly a snake should weigh at a certain age because it depends on how they're fed and how often. My girl went from ~100g to ~250g in 2.5 months without missing a meal. I fed 10-15% every 5 days. She was probably 6 months old at that point but it's impossible to tell because I had horrible self-control and bought her from Petsmart.