How Do I Help My Lonely Dog?

I used to live at my mom’s house and I have a male papillon and she has a female jack russel terrior. After living together side by side for a year, our dogs were attatched at the hip they did everything together. I moved out and I took my papillon with me. Now he cries all the time and doesn’t like to go to the bathroom. I am in an apartment so I can’t get another dog. My mom won’t let my papillon stay with her and I need my papillon to stop going potty in the house while I am away!
Questions:
How come my papillon used to be potty trained but now refuses to go outside and waits till I’m gone, then goes all over my apartment?
How do I train my papillon to not be lonely and to not need my mom’s jack russel?
Is it a good idea to let my boyfriend’s parents have him so he can play with their two saint bernards or is that dangerous for such a small puppy?