Oh no how frustrating for you!
Hopefully it should literally be a week or two before you can get him neutered - they can be done as soon as their testicles appear, which is usually around the 12 week mark.
In the meantime, please don't even let them have supervised playtimes. I know it is heartbreaking, but it literally takes a couple of seconds for buns to mate successfully, so there is an extremely high chance that she would get pregnant even with you watching their 'play'. If she did get pregnant then you would need to separate them for 8+ weeks (longer if they had a litter and she got pregnant again before you even found the litter) which would be a whole lot worse than just a few weeks now.
The general advice is that males should be separated from unneutered females for 6 weeks after their neuter, to give time to ensure that they are sterile and to let hormones die down. However, if he is neutered as soon as his bits drop (so check daily, and ring and get him booked in as soon as you find them!) then you may find that you are able to reintroduce them after a couple of weeks. 2 weeks always used to be the advice, then it started to grow to 3, then 4, then 6. It is erring on the side of caution which in most circumstances is sensible but as you have an existing bond to think about, I personally would suggest that around 3 weeks would be reasonably safe.
Good luck xx