Why is my male guinea pig losing fur around its bum? What to do?

Thanks for your question about Atom. I didn't see how old Atom is, but many times with our middle aged to older guinea pigs (2.5-7 yrs old) that are not castrated, they will have excessive production of material from the gland around their rear end. This is called the caudal gland, and it produces scent in male guinea pigs. When this gland material becomes excessive in amount, it can get stuck to the hair and make it fall out. If this sounds like it may be the case with Atom, the treatment is to wash gently with a pet safe shampoo, and you can clean the material off. After that, it would just be waiting for the hair to grow.
If this does not sound like what Atom has, there are other reasons for hair loss in guinea pigs. The most common are fungal infections (ringworm), parasites (such as mite and lice), and bacterial infections. These would require a veterinary visit for diagnosis, and to get treatment. The testing for these issues is very straightforward and require a sample of hair and scrapes/samples of the skin. Please do not give Atom any human medications at home, as many of them are harmful for our furry friends.
I hope this gives you a place to start, and that Atom feels better soon.