When I was raising Gerona, the only time she would have accidents is when she couldn't make it to the
litter box in time (in her opinion). Sometimes you honestly just have to remind them!
I would take Gerona right before bed time/after a hard play and put her in her kitty box and take her paw and scratch it a bit in the litter. Then she usually went and that was that.
You could also try putting a small kitty box in the room he's using the bathroom in. He could be like Gerona and just feels he can't make it to whatever other room the kitty box is in to potty.

(Even if it's next DOOR.)