I know males start losing hair where their scent glands are on the top of their heads, but my male is losing hair where his "eyebrows" would be.

That looks like over grooming. Gliders can can do this to themselves. Most likely if they're bored or stressed. Sometimes it can be done by a cage mate. My Lulu was high strong and easily stressed about some things,I referred to her as nerotic, she over groomed. I found that after she had routine out of cage play, she'd grow it back. My Ollie is doing it slightly now since he's lost both cage mates, though he's eating well and his activity level is unchanged.

Most likely not a concern, just try out some different things to prevent boredom or stress. If other areas of his behavior changes or weight loss, check up with vet would be highly recommended.

thank you! I will try taking him out more often and removing things from the room that may cause him stress.