It's all a GAME for the youngster! And the senior kitty hasn't put him in his place.

Have you tried using a water squirt bottle to let Boo Boo know this is NOT acceptable behaviour? Keep the bottle handy, even hook it on your pants waist band for the weekend while youa re home.

I'm not sure WHY Tigger has let this go on rather than giving him a swat and letting him know to knock it off. Maybe she is not a dominant female.

Has Boo Boo been neutered? If not, then get that done, that will help.

How did you handle the introcution between the 2 of them when Boo Boo arrived? You may want to re introduce them. Keep Boo Boo "quarantined" in a room for a few days. Let everyone calm down. Then slowly let him be around the other cats moer and more, at first only when you are home. Maybe keep him on a leash and harness for a bit. I prefer the H harness for cats; mine slip out of the Figure 8 style harnesses.

Introductions can take a week or more before the cats can understand how to live together. you have a weekend, consider starting it now!
Good luck!