If he is all there, but just small there should be a chance that he is going to be alright. He needs to get colostrum, if necessary by stomach tube.
I have had tiny lambs like that, it just means that the cotyledons (of which there is a finite number) were unevenly distributed, and they are what nourishes the foetus during pregnancy. Mine have thrived, but of course there could be some things wrong with him.
I would see how it goes, but not get too upset if he doesn't make it.