I'm sure that what he is getting in bf's more than makes up for what he isn't eating. Typically first foods, even when doing BLS isn't going to add huge calories to their diet so that you would see a large weight gain. I wouldn't worry about it too much, but you could try giving him buttered bread, full cream yoghurt and cheese etc and see if that makes a difference. at this age too they tend to drop off with the weight gains too as they are on the go now and burning it all up.