Seeing that he has not improved from Sunday - I would definitely be taking him back to the hospital or at least a GP. If he is dehydrated they may need to admit him and put him on a drip. Has he had his rotovirus immunisation? If he hasn't it could be that so you should get him seen to.

If you are worried about your supply try to express some milk to keep it up and then you have a stash if you need some extra laster on. Often if they have lost weight through illness, they will eat up a storm when feeling better to help regain that weight so you may some extra to give him.