You already sensed that this isn't good for him, because you are here asking.
A puppy's experiences and interactions should all be positive. Now is the time you are doing imprinting for the rest of his life. What he encounters and goes through now is very important.
If he is anything but a very stable dog, this could trigger some behaviors you will have trouble with later.
Dog parks are really not a good idea if your puppy is being harassed by packs of older dogs.
Norden von Narnia (Hans) 1-15-2012