I'm thinking of uninviting my son and granddaughter to Thanksgiving dinner because he couldn't commit to arriving by a certain time
Is arriving at a certain time that important?
He has the nerve to invite himself over without even checking if it's appropriate. I'm short on space.
You shouldn't expect people to arrive at the exact time. You have to think about traffic and other stuff too.
Hey, I don't care about his excuses. If he wants to meet my family then he has to arrive on time.
Unless you have really specific needs, it is more kind to allow people to have some wiggle room in what time they come over. Especially on Thanksgiving, there is a lot of other stuff going on and there will likely be a lot of traffic. Thanksgiving is a time to be with family, so you should think about relaxing your demands.