Thanks so much to everyone for your advice and support! I have been reading all the replies and sorting things out while we worked with Joker. The short of it is our trainer was right, at this point his problem does seem to be a training issue related to the fact that he is green and hasn't been worked on a regular basis in a long while.
The trainer was able to get him to cantor on a lunge on a line, fully tacked, no bucking after just 3 sessions.
The trainer then rode him getting him into a cantor with absolutely no bucks at all!! YEAH!!
My daughter has ridden him successfully at a very short cantor twice now. He has a ways to go but I really think with baby steps and patience he is going to be fine.
We also did have a full vet check done and the vet gave him an all clear so unless we are missing something it doesn't seem pain is the issue. My guess is that yes, he is a bit lazy, he is very rusty along with being green, and has gone through a couple of owners in a relatively short period of time and has been asked to work by various people in various ways and is a bit confused about what he is supposed to do when you start asking for the cantor. My trainer is big on praise...she feels the horse seems surprised when you lavish praise on him. She thinks that someone may have really only reprimanded him when he messed up and not praised him for getting it right.
Thanks for the warm welcome “iridehorses”! It sounds like Boomer is a lot like Joker. When my trainer works with Joker on a lunge line she has his bridle on...it helps make him less likely to “tug” away. Hopefully you are able to turn things around with Boomer, he sounds like a great horse!
I'll keep you all informed as we go along...I'm sure it won't always be smooth sailing but hopefully with time and patience we'll get Joker to settle in nicely!