I know t makes more work, but if you're sure he's hungry I'd give him more feeds not bigger feeds - overfilling the stomach is a prime cause of digestive issues in bottle lambs, if it spills over into the other stomachs they can't digest it and it can cause problems.
You say he calls when he hears you - well, so he should. You're his mummy and he's letting you know where he is. If he's calling all the time, even when you are
not about, then that would suggest distress of some kind. Does he sleep?
Can the lambs see and hear other sheep or lambs where they are? If not, can you put them where there are others in sight, sound and smell?
If you have the time, giving him a cuddle when you can, nuzzling him and talking to him, should help to settle him. His mum would do that, and very young lambs do thrive better, in my experience, if you can give them some attention besides just feeding them in the first couple of weeks.
He'll still call, mind