Dogs genitals bleeding after trying to hump female dog?

I’m watching a male dog and on my page it says dogs will be around other dogs. I was watching him, and my sisters female dog (along with my two pups). He kept trying to hump my sisters dog but we kept him off of her super well. Once they picked her up… we noticed little blood drops here and there. I checked my unspayed female pug and she’s not in heat, then my sister checked her female dog and she’s not. So, I checked my male dog and the male dog we are watching and his tip has some blood. Wth?! This is the first the blood spots have come up since his stay. Could he have somehow injured it when trying to hump the female pup? What can I do to help him heal? I plan on bathing him tomorrow, but I feel really bad for the little guy. No visible injury from what I could tell? So, did the inner “red rocket” get injured??? Obviously I’ll let the owners know, as I know it can be a sign of a uti and other issues as well. But I don’t think that’s it? He’s been fine the entire time until we had to watch my sisters female dog for the day. I’m stumped. 😭