YANBU to think the old man should not have told your son to be quiet. YABU to lose your temper and behave in a crass and common way, completely lacking any dignity and losing the moral highground by ranting and using foul and abusive language towards him. Hope you feel like a bit of a prat now. If I had seen it I would have looked right down my nose at you and sided with the old man, however grumpy and unempathetic he was being about your child.
Anyway - this happened to me once in Sainsbury's tea, having tea with a friend after school, 6 children aged from about 4 to 11 between us. They were not being noisy or raucous, far from it. Sitting nicely, eating nicely, chatting and laughing at a perfectly acceptable and normal volume. The old man on the next table leant across and said (quite calmly and politely) 'could you please ask your children to be quiet? they are disturbing my meal.'
My friend and I looked at one another a bit
and we said (equally calmly and politely)
'No, actually. We cannot. Sorry. They have a right to chat as much as anyone here. They are being perfectly well behaved, not at all noisy considering there are six of them, and it is a public place. If you want total peace and quiet you are welcome to move to another table, or perhaps you should just stay home in future.' We smiled tightly but firmly, and he said no more about it.
That is the way to do it.