You could try something like: "I respect your time and boundaries. I know I need to adjust my expectations for our time spent together due to your change in schedule. It is easier for me to be understanding and supportive when you directly tell me up front that you are simply not free next time. That prevents me from planning and feeling disappointed later."

You may also want to reflect on why he says yes and later cancels rather than just saying "I can't" in advance. Perhaps you don't know why. Were you always okay with "I can't" previously in your relationship or did that sometimes lead to an argument? I'm just throwing out ideas. It's great that you are looking at both sides of your relationship. Very important. Well done!!!