Do you have a joey rejection kit? I'm assuming with this being your third set of joeys that you do?
He will need to be fed every 2 hours for the next few days. Do you have Wombaroo Milk Replacer? If not then puppy milk replacer can also be used, but ideally Wombaroo is best as it's designed for gliders.
He will also need you to stimulate him to toilet. Do you know how to do this?
Joeys cannot regulate their own body temperatures so you need to keep him warm. Not just wrapped up, he ideally needs to be on your skin or on some sort of heat source. A snuggle safe heat pad or a hot water bottle that is covered and regulated to make sure it is kept at a safe, warm temperature.
If your female has rejected her third litter then I think we will al strongly suggest that you neuter the male, these are clearly not working out as a breeding pair and it is unfair on all of the gliders to allow them to keep breeding.
Good luck with this little guy but if you haven't prepared for rejection before now then the odds are strongly against you at his age :(