Did the vet not advise? I would milk off some from the dam and tube feed the lamb - I guess the vet gave AB's? I would probably give 50 to 100ml (depending on size of lamb) every three hours or so. If the lamb has sucking reflex, then bottle (with milk/colostrum milked out from the ewe) would be better. As long as the milk going into the lamb is the dam's own, she will let it suckle once it gets strong enough.
Did the lamb get any colostrum?