I agree that Gettingby said it very well...this is a choice versus a boundary, as far as I am concerned. A boundary is something a little different (like "I will not stick around and let you abuse me" or "I will not spend time with you if you are intoxicated" or "I will not pay your bills if you lose your job because of your actions")
Picking him up is an act of kindness. If it conflicts with your plans (like a meeting, very important) than give him notice to make other arrangements. That is all you owe him. Some notice. And press on with meeting your own needs. If the other evening works for you, keep doing it. You don't need a reason nor justification. Just the confidence to make your own decisions and stop worrying about his potential responses or hurting his feelings or putting him out. He's a grown man - he'll find another way all by himself!