Is the lame leg hot at the joints, look between the toes too incase of scald. Does the lamb take a bottle, try you may find it will take it with gusto, or make up an electrolyte solution , the are many on line. You do need to see vet very soon for antibiotics if it is joint ill , hot swollen joints, check navel. The poo imho says the lamb needs more roughage, offer some lovely soft hay and get some low lying water for him to drink to if he can't reach waterer. Perhaps his mum has dumped him, we have a few that have ended up as mollies like this and they have taken on the role with gusto. Hope he ll be ok x