Supplements tend not to work “one time only.” You need to feed them over time, especially the magnesium ones.
Various pharmaceutical tranquilizers work on a one time only basis. I have never used these but you could ask your vet.
I would suggest in addition also doing a lot of agility/ obstacle/ bomb proofing work with him before you leave. The work here is twofold. First, it gets the horse used to various stimuli. But more important it establishes that you can be a quiet and safe guide around new stimuli and the horse will look to you for guidance.
This step gets skipped alot with dressage horses but it’s just as important as for trail riding and western horses because you never know when an empty coffee cup will blow clattering down the bleachers as you pass!
A couple of months ago I took my horse to a local fall farm type thing for a tricks performance. Afterwards, we had her in a paddock behind the bandstand, and she was fine until the Johnny Cash cover band started up. She started getting reactive, which is very unusual for her, but when I came to get her, I was really pleased and delighted that she came straight to the gate to ask me to get her out of there. I didn’t have to chase her down and calm her down and struggle to get the halter on. She knew that I would take her out of the situation and she was up on tiptoe but behaved herself walking to the back of the property and the trailer. Having a horse that looks to you to solve problems like that rather than just losing its mind is really wonderful. It felt like a milestone.