Thank you, mle-sober and reed, for your replies too. I must have been posting again at the same time you were.
Would we be "beating a dead horse" if we talked to my brother again and let him know that we EXPECT him to catch up on his rent right away, BEFORE he goes drinking, buys pot, etc? (I suppose I really wouldn't say that. It's probably passive aggressive.)
We just talked to him about a week ago, and heard his plans for getting caught up on his rent. In order not to have him feel bad, I told him that because we are planning to move, he would have to look for a place too, but I told him he could have until October to do so, as I thought this would allow him time to save up enough money for a security deposit, etc. (We are looking at places to move, possibly, and very casually, but I thought using this as the reason for him to move would make it so there were no hurt feelings.)