Hope your appointment went ok today, Corrie. Sometimes small babies do struggle to put on weight early on. It's not what people expect, because often even medical professionals tell you they'll be starving hungry and catch up (which is usually, but not always, the case).
I agree with previous posters who say that if he is alert, and meeting all milestones, try not to worry.
Also keep doing exactly what you're doing. Feed often or on demand, don't try and stretch him to make him hungrier for a feed.
But equally, if your DS is referred, try not to worry about that too much. (Easy to say; the hours I spent worrying . . .!). Even growth specialists don't like to intervene until the child is four. And in a way, it is better to have any fears allayed by a specialist, rather than be constantly worried by the health visitors umming and ahhhing and not actually doing anything.
There are little, non-invasive things that can be done, like giving high calorie formula.
I hope you got some reassurance today.