I'm honestly not sure what to suggest here. Border collies do need a lot of exercise - they're bred to be working dogs, and a day of running about herding sheep is quite literately a walk in the park for them.
So yes, being a family pet, not getting out for a run enough, it's bound to frustrate them.
Set against that, you shouldn't beat yourself up too much over this. As you say, had you not taken the dog in he'd have been put down by now. You tried, and you gave him a year he otherwise wouldn't have got. I'd say you should either commit to exercising him much more, regardless of the weather and such, or take him back and give him the chance of a more suitable home. But I can't say which of those you should do.
Having said that, if you were in the UK I'd be PMing you right now to come and visit him.
We lost our dog about eight months back, and we do miss the furry little maniac...