My best friend is dating my son, the friendship has changed, and i'm feeling resentful, how do i handle this?

If you son is of age there is nothing you can do and should distance yourself. Any opinions or interferences will drive them closer together or draw you into any problems they have. Let this die out on it's own and forget about your friend. If it lasts then it was their decision without your influence and you will still be fine, as fine as can be expected.