Ok my advice is...
He's a 5month old GWP, he should not be off the lead until you have a 100% solid recall. I suggest you keep him on the lead from now on until you sort this, otherwise you will NEVER get it sorted.
He's a gunner, they have inherent tendencies to hunt and explore.
I agree with what Jackson said, get a long line!
BTW, what are his lines out of interest?
