
Try giving the treat once he has finished. As my pup was actually performing I used to say " good boy, busy", which was the word we used, and gave him the treat as soon as we could see he was about to finish weeing. I also used to take him into the garden on his lead so he was unable to wander around and he fairly quickly learnt that lead on in the garden meant it was a toilet expedition :)
I would divide the crate and make it smaller so that he hasn't got the room to wee outside of his bed - most dogs will not toilet where they sleep. If he has room to wee in his crate, but not in his bed, he will make the most of it.
Perhaps it would be worthwhile for a few nights to get up to him every 2-3 hours, as you did when he was younger, which hopefully won't give him the opportunity to have an accident in his crate. Sometimes you have to take a few steps backwards in order to progress. :)