So I felt like he needed to "loosen up" and learn to have fun - instead of associating loping with just work. I thought if I could show him that loping could be fun he wouldn't buck at the transition.
Something that my guy loves to do is just run around on a loose rein and stretch out forward & down (ha, and it'll do wonders for making your seat more independent!). As a reward at the walk, trot or canter, I will drop my contact and let him stretch out and really move. This is a great de-stressor for him. Why don't you try relaxing and making it fun? Ask him on a loose rein (even if it's not pretty and he runs from the trot into the canter-- it's still a buck-less transition and forward) and just have fun! Plus after all the hoity-toity dressage work we do, I feel like a cowboy thundering around the arena barely neck reining and it's quite fun.