Help!

Before my SO and I started living together, my son and I co-slept. We had done this since he was an infant, but he had no problems sleeping on his own in the past. Heck, he'll do it for his grandmas, even.
My SO gets home from work about 11, 12 when my son is already in bed. Typically about 3, he has to put him back in his own bed.
Last night, my son learned a little trick. Crawl into Mommy's side of the bed. I was too exhausted from work myself that I just let him stay there all night.
It doesn't exactly bother either of us, but I miss us having our own bed - as selfish as it sounds.

Is there an easier way to get him to sleep in his own bed all night?

Keep putting him back in his bed anytime he tries getting into bed with you. It's exhausting but stay consistent and he will get it soon enough.