Is it definitely Bolton? I mean, the murder is seen from Catelyn's POV and she would know Bolton, but doesn't describe the murderer as him. Could the 'Jamie Lannister...' bit not just be something Bolton instructed one of his men to say to Rob, or could the man not just have overheard Jamie telling Bolton to send his regards to Stark, when they parted ways at Harrenhall?
IMO it's as close as possible without explicitly saying it. Roose Bolton gets up and leaves before the massacre begins and then at the very end, a man in a pink cloak stabs Robb and says "Jaime Lanniser..." Now yes, because of the POV structure, it's not explicitly stated "And then Roose Bolton appeared!" but given everything that happened before and after, I think it's fairly reasonable to say Roose Bolton personally killed Robb Stark.
And yes, Deckerd, as Captaindemotion said, we're discussing the scene from the book, not the TV series. The TV series won't be on the Red Wedding til later this year.