Just a differing view here! Collies and collie crosses as you know are fantastic busy highly alert dogs. They are constantly looking, listening and thinking.
I know the saying a good dog is a tired dog but sometimes this may not be the way to go. A collie when out and about will be bombarded with things to worry about, have to herd, have to sort out, have to understand.
Turig Rugaas has done a study on dogs activities and has a lot to say about exercise overstimulating dogs. http://www.canis.no/rugaas/ website here. She talks about the need to allow dogs to chill and have time to relax and then they often become calmer.
interesting article here on the same idea
Of course all dogs need to be exercised but at the moment agility may not be the way to go. A quiet walk on lead may actually be more relaxing and calming for an anxious dog than hours of ball chasing etc.
I would certainly work on rewarding calm behaviour. So waiting for a down and clicking and treating that. I would also up clicker training as this is a calm tiring activity but not likely to overstimulate the dog activity.
Can he have an area where he can feel very safe, away from all stimulation, eg quiet maybe not over lit etc.
Also have you tried the Look dog approach, this works brilliantly with clever reactive dogs. He looks at a dog you click and treat (whatever the reaction) very soon he will look at a dog and then look back to you immediately for the treat. You will have to work on the correct distance if you are too close then you just need to increase the distance between you and the dog. There are other posts on here about it.
Be prepared that he may never be a totally sociable calm dog but you should get to the stage where it is quiet at home, and you can manage any situation out and about. I do totally understand how tiring it is to have a reactive dog and until you have lived with one people can not begin to understand. However a lot of his behaviour is because he needs to quard and protect you so indirectly comes from his love for his family.
Have you said what he is fed on - this can certainly help as well.