First step is making him scared of the doggie gate. LOL I can lean a doggie gate up anywhere and it's like an electric fence with razor wire; no one dares touch it. Both Bea and Bo knocked it over once and that was it; the doggies gates -- all of them -- were officially EVIL and trying to eat them. This has worked in my favor.
There is no shame in separating the two for periods of time. Oliver is going to have to learn to occupy himself -- with a toy or sleep -- and the tireder they get the KOOKIER they get it seems. Bo is that way when his cousin Olivia comes over. He is RELENTLESS. He doesn't see her but once or twice a year and when she shows up, it's as tho she's here JUST FOR HIM TO CHASE. Ollie enjoys it however so it's kind of hard to police with her egging him on like she does.
Maybe now would be a good time to teach "Place"? That's when each dog is to go to his/her blanket/bed and just stay on the blanket/bed regardless of what's going on around them. It would be a great leadership exercise anyway. Pack leader dogs often "herd" their pack places and hold them there as a illustration of their leadership.
Taking him for an extra long daily walk would definitely help. It sure helps Bo. He and I go walk -- vigorously - and then come back and get Bea for the lazy strolly part of the walk. Same leadership rules for both -- just a slower speed with Bea. Maybe you could also look into agility class for Oliver? He could be one of those dogs that just need a job and the mental stimulation that comes from an activity like a "job".