What does your trainer do when you horse "learns something quickly"? Does he reward the horse by moving on to something else or by stopping work? Or does he continue asking for the same thing trying to perfect it?
As someone with a horse who gets bored quickly...once he's got it, or he's started to get it, we either end for the day (like BeaSting mentions) or we move on to something else. We also spent a lot of time figuring out what he thinks is "fun" so we can reward him that way too.
Too much drilling over and over and he just starts to give me the horsey finger (not bucking or anything, just getting lazy and uncooperative).
Is is probably hard for a horse that reached maturity being able to more or less have full control of his day, to suddenly be in a program where he has to focus on the demands/wants of someone else.
I would tell the trainer that you aren't in a rush, and would rather see the horse work for short sessions for a while, and see if he can slowly extend the horse's focus, rather than work him to the point he can't mentally take it and resists...which is what I am guessing is happening.
Often as a trainer, there is pressure to achieve within a time frame/budget, but this horse may require something different.
With the feral horses we started, many of them older, we put them to work right away, going in trail rides about the fifth ride, in a group of other horses.
We didn't really start working on basics and refining those, drilling in the arena, until later, once they were going on and had interest in doing things with humans and had forward and straight down pat.
Those kinds of smart, older horses you don't talk into doing what you ask, they already have a mind of their own you have to work with or they learn to resist.
Trying to handle them like young, immature horses, teaching by repeating time and again comes later, once they decide they after all will work with you, not be resisting.
It is soooo much easier, when we are talking about installing a good work ethic with minimal resistances, to start when youngsters.
Ask your trainer what all he is doing with your horse and maybe suggest starting him with something interesting, ponying colts, some pole grids, trail rides, series of tricks, etc.
Is the horse pissed at not getting a release from good work, or is he just pissed at being bored because he learns too fast? If the problem is #1, the trainer needs to start using more praise and release so the horse knows EXACTLY what is right and wrong. He will start to look forward to that and will most likely become easier and less pissy.
Is the problem is #2, keep introducing new things! Doesn't mean skip steps of training, it means keep it interesting. If you're working on, say, leg yields, don't just go down each quarter-line and leg-yield to the rail. Leg yield from the rail in, back out, and then canter a long side and change direction. Repeat with variations.
If he gets bored, keep it INTERESTING. Do things that don't overload his brain, but keep him thinking and on his toes. Desensitizing could also help. Bring in some pool noodles. Work on, say, leg-yields for five minutes. Then desensitize him to the noodles for three minutes. Repeat. Things like that, that keep his attention.