Neutering him now is the perfect timing. You should be able to return him to the colony very quickly. I have kept my boys alone the first night after the surgery then returned them to the colony the next morning to sleep with the family.
If you wait again until the joeys are OOP - when he is really needed to care for the joeys - it is likely that you will have MORE joeys on the way again.
Gliders usually mate again as soon as a set of joeys come OOP as they probably did this time.
Once dad is neutered, you can keep the first joey with mom and dad and she will also help with the joeys. Family colonies are wonderful to watch.
If either or both of the new joeys are boys, they will need to be neutered before they are 16 weeks OOP so they can also be kept with the family.