Hoping the lamb is still with you... A drink of glucose and water, or Pfizer Scour Formula or Rehydron - anything that gets some sugars and salts into him. If he isn't taking anything, keep a check on how his breath smells. If it goes 'ketoney' his rumen is in trouble. If you'd already given him PSF or Rehydron and he goes ketoney, you could try Twin Lamb drench, I guess - it's a similar circumstance, system overload, rumen stopped. Like 'Lamb Boost' or similar, you are probably not supposed to give it other than in the specifically prescribed circumstances, but in your situation if the lamb is fading and not taking nutrients in, I would try it.
It's important a sick sheep doesn't lie on the rumen so it's best to prop them up with a straw or hay bale either side. I once made a kind of sling using a hurdle balanced flat on stacked bales, the ewe's legs pushed through the rungs (padded with straw for comfort) to keep her rumen upright and her legs from getting deadened by her weight on them.