Answer

By Your Horse

Vet Gil Riley says:

Although stringhalt is unwelcome, it’s not usually considered a serious condition. It’s very unlikely to get any worse with time and won’t in any way make your horse unsound for work.
Stringhalt is the result of some imbalance in the nervous supply to the hindlimbs, but it’s not a particularly well understood condition. There aren’t any supplements that have been proven to directly help and currently the only treatment is a surgical approach, where extensor tendons down the front of the hindlimb are sectioned in an operation, carried out under general anaesthetic. The condition has to be severe before considering such a drastic approach.
As you’ve noticed that the stringhalt worsens when he’s tense, one supplement that might help is a magnesium-based calmer. These have been shown to reduce mental tension in some horses and I think it would be worth a try in your case.
The best calmers in my experience include both magnesium and tryptophan. Try a well known one from a reputable company. If it’s going to help, you should know within seven to 10 days.