I'm not a shock collar person, and a lot of other people have already said what I would have about the other options.
So let me say how great I think this is - and offer another idea to be working on. I know he didn't hold it long enough for you to get there, but the fact that your dog was running at another dog and reacted correctly to the sit cue is awesome in my book. Have you trained an emergency down at all? I've known several dogs who have had fantastic ones, and I think it is a great thing to have in the toolbox. Particularly with the herding dogs I've known who had really well trained ones, it can sometimes be a good alternative to a recall that may not happen because they are SO focused on something moving. Having them just drop into a down in place keeps them safe but doesn't require them to turn away from the thing they are focused on. Obviously they are supposed to hold it until released. But the fact that you got the sit in that situation makes me think it is something Slick could learn fairly easily, and might be a good 'emergency break' for those times where the recall isn't happening.
So let me say how great I think this is - and offer another idea to be working on. I know he didn't hold it long enough for you to get there, but the fact that your dog was running at another dog and reacted correctly to the sit cue is awesome in my book. Have you trained an emergency down at all? I've known several dogs who have had fantastic ones, and I think it is a great thing to have in the toolbox. Particularly with the herding dogs I've known who had really well trained ones, it can sometimes be a good alternative to a recall that may not happen because they are SO focused on something moving. Having them just drop into a down in place keeps them safe but doesn't require them to turn away from the thing they are focused on. Obviously they are supposed to hold it until released. But the fact that you got the sit in that situation makes me think it is something Slick could learn fairly easily, and might be a good 'emergency break' for those times where the recall isn't happening.