You known Lochlan was alpha-cat since you were brought home as an itteh-bitteh kitteh. Why, after 4 years, do you now feel the need to make a power play? Keeping him from coming upstairs where the litter boxes live is very. Not. Cool. Knock it off or you're going to become a barn cat.
Dude, you outweigh Jasper by about 10 lbs. Just beat the stuffing out of him and be done with it! If you want, I'll even hold him down for you. In the meantime, Mommy brought you down your own litter box; USE. IT.