Regarding the Ace - Dr. K will know better than me - but I've been told it will not work well if the horse is already upset. It can't attach properly as the horse is already excited.
Did your vet mention perhaps using something else to get him calm, then follow up with Ace as needed?
In my experience, if you can give the Ace when the horse is calm - you only need to 2 or 3cc to help a horse stay relaxed.
Since Jasper is already upset, even 10cc might not be able to work.