Is he going after him in a "I want to play!" manner or an "I want to eat you!" manner?
If he wants to play so pestered the senior (which it kind of sounds like), I would say that kind of sounds like a zoomie. If its only during a certain time of the day, crate him during that time, or have a quick play session. My dog always got zoomies at 8:00pm, so I would do a quick training session with him or play with him a bit to fend it off. When our yard was fenced I would simply call out "Go outside!" run up the stairs, fling the door open, and let him race around the yard while I stood in one spot "chasing" him to burn that last bit of energy because I'm lazy, lol.
If he wants to play so pestered the senior (which it kind of sounds like), I would say that kind of sounds like a zoomie. If its only during a certain time of the day, crate him during that time, or have a quick play session. My dog always got zoomies at 8:00pm, so I would do a quick training session with him or play with him a bit to fend it off. When our yard was fenced I would simply call out "Go outside!" run up the stairs, fling the door open, and let him race around the yard while I stood in one spot "chasing" him to burn that last bit of energy because I'm lazy, lol.