Thank you both so much for your replies. My instincts say everything is okay but I don't want to be blase and find out late that things aren't as good as I thought IYKWIM.

My MCHN rang to check I was okay today- so nice of her! We're going to visit her and use her proper scales tomorrow in case her portable one was dodgy. Just for peace of mind to know for sure what his weight is, but she's telling me not to worry at this point which is good. Just going to keep pumping and topping up after feeds to make sure I have good supply and he's getting plenty in.