D.K.
3-1/2 is old enough to understand a lot about surgery and doctors. If you haven't already read it together, Curious George Goes to the Hospital is a helpful book. I think you can really try to explain what will happen in simple/lay terms and then just ask DS to explain back to you what will happen so you can see what he doesn't understand.
Best of luck and I hope DH has an easy recovery.
D.