I don't know why, but it's not unusual for horses to be much more fussy about picking up their back feet. How to handle really depends on how hard they want to kick out. If it's just a half hearted attempt, I'll just pull their leg out to the side a bit and hold their hoof up by the toe...they can't get much of a kick in that position (the goal to not let them get their hoof down until you say it's ok). If they persist, a good smack on the butt usually gets their attention that they need to cooperate.
Try to start with the lead rope. Slide it around the back leg and pull it up off the ground. Just before he goes resistant and wants to kick out or pull away let it go back to the ground. This way you teach him and staying safe yourself. I used it on both my youngsters.
BTW, interesting enough I had lots of issues with front legs on my qh, but back ones were very smooth. Paint was other way around. I started them both from zero (they were unhandled).