I'd shorten the tunnel up, see if you can have someone hold him at the entrance (so he has to take the tunnel) and when he's let go and AS he exits that ball gets rolled out fast and straight from the exit.
Only lengthen the tunnel IF he's pulling to get loose and thru with it the current length. Then you can lengthen shortened as his ATTITUDE shows you to progress.
When we add a toy to any/all training it really takes care of all their anxiety. So they aren't doing it even though they are worried................. we want them to WHOOHOO their way thru with no worries at all. The tunnel is a perfect place for that fast fun rolling ball to get them to tear thru, out and away!!!!!!!!!!!!!!
thanks for the tips Maggieroselee. i had it full length for my 4yr old who zips right thru it, and as they were playing my youngster decided to go thru it. yes, he loves balls, and i will use your technique. hard to teach tunnels alone..