Yogi is 3 months old, Gracie is 9 months and Boo is 6 years old. Gracie pestered Boo unendingly when we got her. Boo is a great dog and only once did I see him jump at her and growl a little. Boo quickly realized he could get on the bed when he needs a break from her. Also, we were very careful to not let G torment him. Play yes, torment no. Then we got Yogi a few weeks ago and now he's doing the same thing to G. You'd think with their ages being so close that they'd both just run and play all day. But Yogi gets after her pretty rough some times and we have to keep a close watch on them. G can get on the couch to get away, but she has figured it out like Boo did.
Besides the obvious - be sure they are both neutered. Is there anywhere that Buster can go that the puppy can't? If so, try real hard to get Buster to realize that so that he can regulate when they play. Some puppy classes to help establish boundaries for the puppy and limitations on how they are allowed to play should help. Supervision is tops right now so that the puppy learns the rules. Perhaps leave Buster home 1 or 2 days a week so that he'll have a break and so that your folks can work with the puppy. You should be able to work thru this, it's just may take some time and effort. Hang on to this one thought...he won't be a puppy forever and he should settle down some as he matures.