@Kferg - No, I haven't. He/she had a full vet check before I got him, but I'm planning on taking him for a yearly checkup, which is soon. He doesn't have a bald spot, no - which is another reason I suspected he was either neutered or a girl. But then the breeder said she had not neutered him, so I though maybe he was a late bloomer. It's all been a bit of a mystery. I think I'll make an appointment with the vet and have him clear everything up once and for all. Oh and his/her name is Skittles - luckily it can go either way gender-wise :)
@Candy - thank you for the pictures - they are nice and clear and easy to compare, especially with there being no fur to cover it up yet. I see what you mean - it looks different to both of those.
Thanks SO much to everyone for their help! Relieved to hear it's a gender issue rather than a health one. I appreciate all the glider community help :)