
Sounds like perfectly normal terrier play - pups especially love rough play

It is your job to decide when they stop and to separate them if necessary to ensure the pup gets enough rest - a tired pup will play harder and bite more, just like a tired child

Your older dog should also have somewhere to retire to so he can get away from pup when he's had enough or just wants some peace

I've always found that young pups get plenty of rest when given their meal
after play sessions, it's easier to sleep on a nice full tum
It's not what you say, it's what you DO that counts...