Partner cheating?

Hi everyone so my partner has been acting a little strange lately. He goes out drinking till 6-7 am and today I found a scratches on his back to me they look like they're from sex and its definitely not from me. He says he doesn't know where they come from/ did it himself but I've felt his nails they're so short there's no way he did it himself. What do yall think?

EDIT

thanks for all the comments when he's coming home from drinking he's not even drunk and says he only had 3 drinks. Definitely not blacking out and not remembering