My fracture is in the middle of my forearm and both radius and ulna are severed. my surgeon does not want to do surgery. is this a risky decision?

It depends: Mainly how well the bones segments are aligned. If it is good, no surgery is a good option and immobilization is all what is needed.

Not necessarily: There are risks to surgery and to casting alone. If your alignment can be maintained safely in a cast that is a fine treatment plan. If the alignment is not good or is very unstable then surgery can be helpful. Other factors such as your general health, skin condition, etc play a role in the decision too.