Long trotting!! I own 1D barrel horses and I do it with them in my arena all the time!
I would try walk, trot, cantering over ground poles and small jumps to get them using their hind. Also going up and down hills and through uneven ground (somewhat like some cross country obstacles) to really get them moving and getting stronger.
Collecting your horse, keep your horse in the bit, only then u make sure he is using his hind end . And once used it will be strengthen for sure
I agree with lily and i would like to add an exersize. I like to take half of my arena and weather im in the trot or canter, slowly spiral twards the middle then slowly proseed outward again
To strengthen my horses hind legs I did a lot of Walk to trot transitions over and over again, I found it made her use her back legs to move into the trot transition more.
Hope this helps😊