First Valentines I'm actually dating someone, sadly it's still with my bf whom we're most likely splitting in a month. Im still worried how to say it when I do move and how he'll react.
Tell him in black and white, as short as possible with as few words as possible. There is no reason to make it long, drawn out. Do what you need to prepare yourself, maybe try writing something down so you can think about exactly what you want to say. You know the situation best, so sorry for the very general advise.
How he reacts.. well, you can't control that but you can control how you feel. People get all up with you 'owe' them something, but in the end you really don't. People don't trade favours back and forth. If he has done something nice, then he did it to be nice not because you owe him (and the other way around).