Age-wise, he wasn't weaned early if you weaned him 10 days ago and he's 9-10 weeks old now.
But I'd agree that bottle lambs generally need to be eating a decent amount of creep before losing the milk. General advice is 1/2lb a day for a healthy commercial lamb, half that for a small primitive type.
Several of us do well with Champion Tup & Lamb. If you can't get that locally, try a pasture mix rather than a pellet; the different shapes and textures make them explore and play with it more, so it starts to be eaten as they mouth it.
It sounds like his digestive system is generally not in good order, probably wants a bit of a reset. You might find it best to take him off grass for a while, let him eat hay and a feed only until his digestion rights itself.
I would certainly speak to the vet, it's possible he has an infection which will need medication. Otherwise, I'd be inclined to bring him off grass, give hay only (the best softest hay you can get) at first, and for 24 - 48 hours, give him PFS or similar to keep his electrolytes right and get him rehydrated. If he brightens and is eating hay well by then, introduce a pasture mix / creep.