Yes, sounds like a good plan. Whenever you are under the impression that H will be looking after X, always try and plan something. So then when he wants to change times, you can say "I have something planned at that time" of course he'll want to know what and you can tell him. Then see what he says. Then if he really can't get out of his thing, say "I'll have to see if I can cancel it, I don't want to let such and such down, but hopefully they'll be ok about it. I'll call you back".
if every time he wants to change the time you have something on, he'll start to realise he needs to let you know earlier and if you're frequently cancelling things you will be in a better position to say.. "it's important that we stick to agreed times as I'm letting down a lot of my friends (or work) at the moment and they are starting to get jack of it."
at the moment i think you are always waiting till he confirms his plans before planning anything. however, then H knows that you will always be free to look after D until he confirms what he's doing. so at the moment there is no need for him to tell you or plan things earlier. then you are left waiting closer and closer to the time before you plan anything.
I think the word is "being reasonable" not "flexible".
