Zyklene may be worth a try - shouldn't do any harm.
Adrenalin stays in the dogs body for up to 24 hours after stressful events so with stressy dogs you want to prevent "adrenalin stacking"
eg dog gets in a stressful situation, dog then encounters another stressful situation for him and the stress just build on top of original stress, then a situation that does not usually stress the dog out much makes the dog go over the top as the adrenalin is now bubbling over in his body.
So general rule is to try to prevent the stacking from occurring. So easy to do right !
Generally training, clicker training is great for this as the dog never makes a mistake and is never need worry about making a mistake, drop back on chasing games (probably a dog that is reactive outside is encouraged to chase so that he runs off his energy on his restricted walks but this does mean that his walks are high drive, maybe positive adrenalin rushes but the dog is still highly charged)
Encourage scent games, so instead of chucking the ball, hide it for him to find scent games are knackering for the dog but also very relaxing and brings the adrenalin levels down.
Give him his dark quiet room for a chill out regularly, (is he a shadow chaser?) so even light can again charge him up.
It may not be for you but I would consider changing to raw food nutriment is an easish way to do it - then you can eliminate any additives etc that on a calmer dog may have no effect but could effect an already stressy dog.
There are lots of other strategies but I don't want to bore you if you are already using them.
You should never generalise but if you have a stressy spaniel they are very good at it 