guinea pig purring

My guinea pig when put with his other Male friend purrs a lot and lifts his butt to show what looks like a prolapse or possibly a clogged grease gland whereas when he is sat normally and calm it’s not visible ? I will attach pictures.

So about like 5 days ago i got a guinea pig (male forsure) and the guy who was working at the petshop told me that the other pigs are his brothers. So he also checked them and told me 1 was a girl and the other one was a boy. And today i decided to go back and buy the male so the first pig can...