Will he lower his head and lift his back on a loose rein? If not, I'd go there first.
I was having this issue with a 7yr old mare... Three coaches had me going about it much as you have described... My horse was getting more and more anxious because she hates getting things wrong. This weekend I rode with a different coach and she explained that it is simply too much for my pretty green horse, at this point in time.
So we are simply going everywhere on the buckle, using flexion to encourage the long and low, then leaving her alone as long as she will stay low... Flexion when she comes up.
As she was getting this concept she was starting to move onto the outside rein, on her own. I didn't really have to ask, it just came along as part of staying balanced.(like your guy, just a few steps at a time... But millions of miles better than where we started at the beginning if this weekend)