Mobile invisible fence for dog that likes to wander too much during hikes?

Hi,
My brother and I hike quite a bit and he has a large mastiff family type mutt who is sweet as he can be but has a tendency to wander. On our most recent hike he wandered off and disappeared and pretty much scared us to death (was hunting season) and we literally were running around in the woods on a night with zero moon and found him around 22:00.

Of course we can use a leash, and god knows we have been trying for years to get him to heel/stay close by but neither us nor the two trainers we have tried have been able to make any headway. We just think that a leash or having us yell "Heel" every few feet is less fun for us and less fun for him (we think) so we were wondering if there is something like an invisible fence device that can give him the freedom to run off 20ft or so but remind him to stay relatively close if he strays/lags too far.

Thanks in advance!