Just been reading your first post again and I'm wondering about his exercise regime? You say he goes with you on
most of your runs. Does that mean that he goes out with you every day or only on some days? Is he on lead just running alongside you. I've never run with a dog (never actually run without one either!
) but I feel that a dog gets more stimulation if it is off lead, able to follow scents, practice recalls, play with other dogs or with a ball etc. rather than just running along keeping up with a running owner. If he isn't going out every day, preferably more than once, then maybe the problem is that he isn't actually getting enough exercise? Are your parents able to take him out or do some training games with him to occupy his brain?
I think Maddy has made some excellent points re his previous history and also going back to square one and leaving him for very short periods and building up. This means it may start with just a couple of minutes etc. Hard work but I'm sure he will get there in the end.
If your parents are not able to cope the I agree that doggy daycare or even a dog walker to come in once or twice a day would be something to look into. Hope things improve for you soon.