I would stop the milk, give him antibiotics and get electrolytes into him; Pfizer Scour Formula or Rehydion are both good. (Both are for cattle really, but work fine on lambs.)
If it's watery mouth, antibiotics should sort it. He won't have much appetite until the a/b's kick in, so just give him enough PSF to keep him hydrated until he seems hungry.
If it's proper scour, milk is the worst thing to be giving him. But you should be able to get him to take PSF. Orajet, Spectam or Scour Halt will all deliver appropriate a/bs. Don't reintroduce milk until the poo is normal.