I just don't think he's going to agree, because it will completely disrupt his life. Now he has his DC living locally and he sees them for 6 hours every Sat. He can have a lie in, he doesn't have to have them overnight, he doesn't have to drive far, etc. Suits him very nicely.
OP is proposing that in future he will have to drive a 6-hour round trip to collect his DC and then the same again two days later to deliver them back. This will involve a lot of money for petrol/diesel, as well as six hours out of his day AND he'll then have the DC for the whole weekend/half of half-term/half the holidays. He's never, as far as we know, expressed any desire to have them for this long.
Plus, how are the logistics going to work? Will you set off with them after school on a Fri night and drive 3 hours south, meet him at a service station, then both head back off with them? If so, that means them arriving at his at around 10pm on Fri night, taking into account the stop/handover and any loo breaks. Or perhaps you'll get up early on Sat, expecting him to do the same? That means six hours of driving on both of your Saturdays and Sundays on his weekends and all that times spent travelling the motorways of Britain for your DC. It sounds shit to me and I can't see any way he's going to agree to that.
I also feel that your not thinking of your DC in all this. You mention YOUR happiness and presumably that is tied up in being with your fiance, but what about your DC's happiness? I can't see that being enhanced by your plan.