Put him on a lead that you or someone else is holding, command SIT, and then throw a mark. If he bolts he will get jolted back. Do this enough and he may get the idea.
It really all comes back to basic OB. In order to really demand steadiness he should know that SIT means that he should sit until told to do otherwise. All the basic OB commands come into play during the retrieve, that is why they need to be the initial focus of a retriever training program.
I love Brittanies, my Dad bred them when I was younger and I have trained about a dozen for upland work. Of all the classic upland breeds they are my favorite due to how they work and their temperment. However, they are like most upland breeds and can be a bit hard headed once their formative years have past. So, it might be a tough row to hoe in order to steady him. Don't use an e-collar, just stick with the lead. Worst case scenario is you have to keep him on lead while hunting. That would be MUCH better than him breaking on the gun and messing up shooting opportunities. I do believe that with enough time spent on lead while marking will steady the dog, it just might take awhile.
just my opinion.......
"I'll start spending less time with my dog and more with my wife when she starts fetching ducks for me"