At this age, it is normal for some bf bubs to go for longer between poos. My (also quite petite) DD went once for 18 days without any problem. Except for the poo-splosion at the end, which thankfully happened at home! She started missing a day, then a couple, and so on. Their digestive system can absorb so much of the bm goodness that there is very little waste.

If you had no reason to be worried before he was weighed, then there's probably no reason to worry afterwards. TBH, I stopped taking my DD to the MHCN and just got her weighed every now and then at a close chemist or when she got her vaccinations. The weight at which he was born may not be an indication of his genetic height and weight, and he needs to adjust to that.