To an extent it's a case of letting them get on with it and sorting themselves out. However, as the old boy is getting fed up and the puppy is getting more and more wound up and excited and ignoring the old boy's warnings, then separating them for a while like you did this afternoon is a good idea.
I wouldn't worry about it too much because they'll probably find their own level of being comfortable with each other eventually. It sounds like your parent's Cocker is a tolerant chap
as all he's doing is the "nice" sort of telling off that most tolerant mature dogs will give to a pushy puppy. When you're out on walks your puppy will meet all sorts and I'm sure there will be some uncomfortable encounters along the way with slightly less tolerant dogs, but hopefully it will still be nothing worse than a telling off and will help your boy to learn how to read the body language of other dogs and moderate his own body language as he matures.
I'm not sure how experienced you are as a puppy owner, so apologies if you know this next bit already
. When you're in the park please take notice if you see another dog on lead or an owner doing an abrupt change of direction when they see you and your puppy! Dogs on lead are usually on lead for a reason (could be old, unwell, deaf, blind, a rescue new to the owner, a nervous dog or perhaps an aggressive dog) so please respect the other owner's decision to have their dog on a lead and give them some space. My 4 year old Cocker is terrified of bouncy dogs that dash up to him, adults or puppies, and will growl and chase if they don't understand his initially fearful body language. So there are times when I change direction and walk away, or if I can't do that I put him on lead to make sure he can't chase.
