I would suggest groundpoles as well, and if you can get him to move more freely...just concentrate on an open/working trot for a good portion of your workout, verses a collected one...get him really moving his feet, that way he really has to pick them up. When he is picking up his feet well during your session, then collect him up for a bit, then let him go again, and so on and so forth. I would also leave ground poles along the sides of the arena, so he always has incentive and focus to pick up his feet.
When my mare gets kind of shuffly, that is what I do with her...although that is rare, since she is a really forward moving horse...Lol!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."