Thanks for your help, Grahams, he's got me confused as well

I did have a look at the profile for S. schall and I think its another possibility, along with the S. frontosus that Silurus suggested. I wish I knew where he'd come from, but I brought him almost full-grown, he'd been handed in to the lfs for bad behaviour

so I don't know which importer he came from. I'm also not a fan of hybrids, so I do hope that he isn't one, but if he is, so be it. I won't be breeding him as I've never seen another Syno here even vaguely like him.
I really do appreciate your help and if I ever do find out what he might be, I'll let you know.