Guys I don't have any experience with springers. I do have a wireless fence with a lab and chessy. They too hate getting shocked and stay home. The only way I take them off the property is in a vehicle. Even if it's just 200 yards away to the woods in the back. That is their safety, loading up and getting to go somewhere fun.
He may have associated that truck with safety, if that's the only way he gets to leave the yard. I like the fun time, that seams as though it would work to me. Also more outings, even off season would help. I think that breed likes to swim right? Just try to make an outing once a week and get him comfortable with it. I don't know anything about check cords. I use the chain and leash from time to time, which I presume is similar. You are just limiting his range and able to teach come, stay sit, heal, hold, flush etc....

It's hard to say what goes through a dog's mind. He could have gotten hit by a briar so he thinks his fence is in the field. Take him to a bunch of different places. Resocialize him and have fun. He will get out of it. You did not make a mistake with the fence. It is better for him to get this little hiccup that can be straightened out than to get ran over, stolen, shot or get into trouble with neighbors.
I hate to say it, but you may need to step up and be the alpha male. He should be depending on your judgement for his safety. Not his irrational thought process of running back to the truck away from you. I am not knocking you at all buddy, or implying anything. I just know until I got this chessy I thought I was the alpha male. I was being way to submissive and just had a submissive dog that tollerated it. Your dog may need a little more guidance, and I have no doubt you have the ability to provide it. Fair, Firm and Consistant. I believe the springer has some of the same qualities as the chessy the only reason I mention it. Highly intelligent but head strong and independant. Semi aloof with strangers. High energy.
So in other words it has got to learn to depend on you to take more of the decision making process on hand so he can get back to the basics and not have a care in the world besides wanting to go out and have fun.
Blessings