Whether Bellfield is innocent or guilty he will never give a DNA sample.
Serial killers will only admit to crimes or rule themselves out if there is conclusive proof one way or the other. Being linked to unsolved crimes keeps them in the public eye.
Stone's lawyers can ask the police to order Bellfield to give a DNA sample. Or the police may already have a sample they could give them. What the criteria is for Stone's lawyers getting the police to do either of these things, I don't know.