Puppy aggression

I have a 4 month old purebred bichon that attacks my 12 year old bichon relentlessly. He is biting, chasing , and constantly tormenting the older dog. My older bichon is passive and will not bite. He just yelps and cries when the puppy bites him. I have used a water squirt bottle, pennies in a bottle and keeping the puppy on a leash. I’m at my wits end.