Do hope you has a lovely day out.
Problem can become that the more you do to help adult children, the more you are expected to do. So your son, is just seeing you as always there whenever he feels he has need of you.
You need to make it very, very clear that, although you are happy to help out on occasions, you and your OH have your own lives to lead - and, except in real major emergencies - that takes priority.
I had a friend who always dropped any arrangement if one of her AC suddenly needed her help. Result was that she ended up with very few friends of her own, and no life of her own. Not appreciated at all by AC. They were always nicer and politer to me than they were to her.
The occasional 'Sorry - no can do', is an excellent reminder that you have done your part in child-rearing and are now carrying on with you life. Their children, their responsibilities.