your own position is much improved in such a short time.
I agree that you may have a bit of "fiddling" happening to his head, and he ends up becoming short in the neck, and maybe tense. it's a common thing, that I think many of us do more than we really care to realize.
but, you can help him now and then become more trusting and relaxed in the shorter rein if you sometimes let him carry the bit well forward, even to where you trot him around a bit nearly on the buckle, and don't worry about his outline, just get him briskly and loosely going forward.
look for the position of his hind leg when it is stepped fully under, in relation to something fixed, like the saddle position, so you can see if he is reaching under himself more.