I wouldn't see it as him being rude if this is the first time he's changed the plans, he might just have different comforts at his place and might be taking the opportunity of having his flatmates away for you both to have a better time or he might just not like being away from home when he's drinking, there could be several reasons (I won't delve too deep into them here, you'd have to find out that for yourself). I'm not sure how much distance there is between your two's houses, but if it was me I would still try and get him to come over to your place.
I'd normally entice my friends with the comforts I have at home such as a games console, or that I have a projector so we can watch films/play games on a big screen. But in this case it's imported beer and good company (?), so you could say something along the lines of:
I've got the beer, you've got the company. Let's make a night of it.
It's a two-way street really, you're providing the luxury of free imported beers and you both have the pleasure of spending some time together outside the work setting. If he persists on trying to get the evening over to his place, just politely decline.
If you want there to be potential for a next time, you could say:
That's a shame because I was hoping we'd be able to relax, have some beers and you'd finally get to see my place. Maybe next time, though.
This way he'll get the hint that you want to be the host and provide the entertainment, but you're also doing it in a way that doesn't burn any bridges by encouraging there be a next time, if and when he decides he wants to come over to yours.
or if you feel he isn't making enough effort in the here and now and someone else is worth the imported beers, you could just say:
I'm sorry but I was hoping we'd be able to relax, have some beers and
you'd finally get to see my place.