Understandably so but truly it's probably your best choice to help him get along with another glider.
If you wait and get another glider before Moose is neutered you will want to get another male, preferably already neutered. Otherwise they will likely breed if you get a female. Which again will lead to possible offspring needing neutered anyways or separated from his parents before he reaches 4 months old to prevent breeding with his mother. If the offspring is a female she will also need separated from the father or the father would need neutered.
They may do ok if Moose isn't neutered, and another male is neutered, it's just a greater chance of them getting along better if they are both neutered prior to introductions.
There are many ways this could go, and I understand the hesitation on getting him neutered but the scenarios above are what you are facing if you get a female for a cage mate.
And if you get a male, they may not get along at all, until both are neutered and their hormone levels are reduced.
I don't want you to feel pressured, but you should be informed is all. In the end the choice of course is yours and you will have our support and continued advice if you should need it.
Introducing gliders is already stressful enough, watching signs of depression only adds to that. If you do end up getting another male, be advised that even if they do get a long for a long time, if you ever bring a female glider in to your family that could cause some fighting with the boys, even if the female isn't in the same cage as them.