S.T.
I laughed when I saw the old dogs is only 2! (My dogs are 15 and 16)! Definitely get them fixed otherwise more puppies will be on the way and unfortunately there are so many unwanted pets already. Taking care of that will definitely calm him down and improve their relationship. It would probably be good to introduce them together with both of them on leashes (so if he tries to mount her, you can pull him off). Spend time with both of them petting them and praising him when he is calm. If he loves to chase things, do a game of fetch to wear him out and also get them to play together without the do focus.
Good luck. Two dogs is better than one, but it takes time for everyone to adjust.