Sorry to hear about the piggie - they can die very quickly and of unexplained reasons. I wouldn't think that the other one killed him - you would see other signs than him being flattened.
You don't say how old he is. I acquired a boy piggie a couple of years ago (he was 5 months old) and wanted to put a friend with him. The vet said that another male of around the same age should be okay but no older. I got one and after a lot of posturing and teeth chattering they got along very well. If you do this don't leave them alone from the outset - I had mine indoors where I could keep an eye on them. The other answer would be to get yours neutered and put him with a female in which case there should be absolutely no problem. Ask your vet about the best age to do this. Good luck and I hope you find a friend for him as they are very social and like company.