Sounds like what was more or less going on with my mare... She was very "forward", but it was always rushy, jumpy, quick... Not a true forward. Her tension wasn't tack related, but every coach we have used has checked that anyhow because that is what the behavior "looked" like. She has never been abused (she had a wonderful, knowledgable home before I got her), she just, for whatever reason, never worked through her tension undersaddle.
She came around really quickly to this exercise, she is 6/7 years old this year, and only been ridden for 2 years... So I put this down to the fact that her muscle memory had not been holding the inverted frame long, so it was easier for her to learn better posture.
I also have that older Saddlebred mare which we work on the same stuff... She doesn't move tense like your boy... Just inverted... You would think that would mean she was easier to work through, but she has obviously been going incorrectly for a longer time, and would rather just bend her neck and head than actually lift her back in order to avoid the real "hard" work. (for this reason she gets lunged, properly, more than ridden in order to help her build muscle and balance without the interference of a rider's weight to make it harder for her)
Along the same lines, I have a paint x, about 8/9 years this year, I have owned him his whole life, used to go well... But I have allowed beginners to ride him for the past three years and he has decided that it is "easier" for him to go around like a bit of a giraffe unless someone experienced is on him to remind him of where he should be travelling.
I guess my point is... All horses can develop holes... Sometimes those holes can really make you scratch your head and wonder what the heck happened to make the horse think it needed to go that route. It doesn't always mean someone without knowledge did something "wrong", sometimes it is more of a case where our bad habits get the better of us, sometimes something we do triggers a long ago memory for the horse, sometimes the horse is simply looking for a way to avoid doing as you ask (be it due to pain, the memory of pain, confusion, or a lack of fitness... Or whatever). It doesn't really matter, as long as you know the hole exists you can work on fixing it.