I agree, sounds really iffy. I presume he is also a Shetland? Poor thing should have been better on his feet the next day or so, but to be honest a basic foot trim would never cause that degree of lameness. If he's a ram 'lamb' his feet would no way be overgrown anyhow. He must have dreadfully sore feet to be eating on his knees and be wobbly on his pins when standing up. Have you checked them yourself? Did you get a sales receipt and pedigree certificate with him? I wouldn't ask to return him, just say you are doing so.
There are checks to carry out to ensure you are purchasing a good tup. A tup is a big investment for your flock and his conformation, fleece quality etc affect the whole of the breeding future of your flock. He needs to strongly carry characteristics of the breed in question. You need to check he has sturdy back legs as he will hopefully spend alot of time on them, is good and strong overall, good teeth, descended and healthy testicles and that he is genotyped (ie scrapie tested) and pedigree registered. These are just the basics and the breeder should have gone over these points with you. They should be severely brought up on all things but you also should have gone with some knowledge of what to look out for I'm afraid to say. Still, essentially, they have taken advantage of you. I should take him back, get your money back and go to another breeder (it's worth travelling the distance for a good tup. PM Ominpeasant. Sounds a good start to me) as you can no longer trust this one. I would also consider reporting said breeder to the Society. I hope they show some remorse at selling you this unfortunate boy.