It sounds like you have encouraged your youngest son's irresponsibility all his life as he probably didn't just start to be now. Just agree to what you are happy doing and refuse to do the rest although it sounds like his dad is happy to have something to do now he's retired. Maybe he could go to his house for part of the day to be with the dog so it doesn't have to come to yours so often.
I'm happy to help out with errands/pet holiday care for my adult children most of the time as I'm semi retired, so not so busy as them as they both work, but if I have a prior engagement they know better than to expect me to change it to suit them!
Medical Ailments (Oct '23) A-Z
To think that London, or anywhere else for that matter, does not belong to any one demographic