Oh, Lucy! I am so sorry you have no answers! At this point, maybe it's time to shift gears as you have stated and consider what is best for Legs. You now know he can't exercise strenuously and even your mom's yard may be too large even if he could self-regulate his exercise, so your present situation may be just what he needs.
I think you will relate to this: One of mine loves to run - she would run herself into the ground if I let her. Unfortunately, she has a tendency to sprain her toes when let loose to run freely and almost ripped a dewclaw off once, so we walk and walk and walk and walk. When she gets "full of it", she does "doughnuts" in the house and she may play crazily with a toy for a few minutes. Does she miss running at the dog park? I don't know, but I do know that if she were separated from me, DH and her litter sister, she would be a very unhappy girl. If I continue to allow her to run at the dog park, she would be in jeopardy every time she ran.
It just so happens that she's also 8 years old - will be 9 on April 1st (yeah, I know, the "April fools" joke was on me when I took her and her litter sister straight off the track!). She's not anything close to the wild child of 7 years ago, but that part of her history is so much a part of her and so fresh in my mind! Do I miss it? You betcha, but if I let my heart rule my mind, she would be in a constant state of hurting and healing so I'm taking the steps necessary to protect her from herself........................ushering her into her senior years so that we can both enjoy the time she has left. In my heart, it hurts to think that I will never see her run again, but watching her limp hurts more.
I know that you will do what is best for Legs. I will continue to pray that you get some answers about what is causing his symptoms.