TMIIIII what could this be?

Ok so my boyfriend got this on the sides of his balls. He's never had it till he started working at his job like 2 months ago. It can't be an std because he's only slept with me. He's been walking a lot more in the hot and he says it rubs on his legs. He usually wears shorts but at his jobs he has to wear pants so it's not breathable as his shorts. They are more tighter around that area. Could this be because of the rubbing.