I regret having my son circumcised
Why do you feel that? just because it's a painful process?
Yeah, I was against it.
I can imagine it must be painful to your son and you regret it. You can't change the past so there is no point it thinking about it now.
I can let my son decide when he is older and let him do it to his children.
What's done is done and I'm glad you took this as a learning experience. It's only natural to have done things we've regretted in the past, and I'm proud that you are open minded for your son and his future decisions!