If his mouth is warm, that's a good sign. If his mouth gets cold, that's a sign of hypothermia and you will need to warm him up and then get some fluid in him. If you can't jug him and his mom, then I would see about tying her up or holding onto her and then having someone hold the lamb up to her teat. If you can't do that, then just watch him to make sure his belly is filled out. If it's sunken in and you can see the bones sticking out to the side of his spine, then he's too thin and not eating.