My fixed male puppy keeps mounting my older female dog. What do I do?

We have two dogs: one male puppy that came just a year ago and a female dog that we had for four years now. Both are fixed and have gone through the surgery, but for some reason, my male puppy keeps mounting my female dog? It’s even more weird because he’s a tiny Pomeranian and she’s a bigger, medium-sized terrier. She just lays down minding her own business, and he keeps coming to do his business with her. I’m worried this is stressing her out; she recently started to poop in random spots in the house despite being trained, and this is only after he came. I keep telling my mom to separate them, but she doesn’t listen. What do I do?