The risk is very minimal. I have lost one rabbit out of around 45 under aneasthetic because she had an undetected heart murmur which only an ECG scan would have picked up.
Your vets can give you a health check and run blood tests before to see if he is at any abnormal risk from the aneasthetic.
You may also want to search for an experienced rabbit vet and ask how many speys/castrations they do on rabbits per year/month etc and what their mortality rate is like.
Im sure they will reassure you.