Hi, Firstly, in answer to your main question, yes, absolutely. Old horses(& dogs... even people... even your trainer probably could
!) can definitely learn new stuff. Just that new 'tricks' may entail 'unlearning' previously learned behaviours & attitudes, which can make it harder, depending on the situation. Eg in this case it may be that previous 'lessons' have caused him to 'shut down' mentally or otherwise get too stressed when confused.
before he had a mental breakdown and completely refused to continue jumping.
Poor boy! Glad you got him out of that bad situation! Of course, it may have been a purely behavioural thing, perhaps that he just got 'sour' having to teach people to jump, but this sort of thing is very frequently due to physical problems - pain. I would consider this carefully, especially at his age & with his previous management, and rule out/treat any physical problems or don't jump him. His 'metabolic issues' due to being overfed & underexercised, for eg could be at the root of it, as in mild laminitis.
but when I try to teach him tricks or anything out of the ordinary, he doesn't seem to understand it at all.
What & how exactly are you trying to teach him?
I decided I wanted to attempt bitless riding, but quickly learned he doesn't respond at all to voice commands, which is pretty import to make him stop! The past few days I have been attempting to teach him that Halt means to stop.
If you want to ride with only voice cues, he needs to have reliably learned that well before
you want to go testing it out! But not that I'm sure you mean just bitless or without any headgear either. By the sound of it and the questions that you're asking, there is a LOT of basic communication you two need to learn between eachother before I'd dream of getting on a horse without any means of control aside from voice - think of that as your Doctorate degree, whereas it sounds like you're barely into primary school with him.
If he's already well trained in basic yielding & you want to teach him voice cues, I'd be just pairing the verbal cues you want to teach him(or halter rein cues, seat cues, etc) with the regular ones he already knows well, to form the association with the behaviour, to begin with. Once you think the horse has made the link, then you can start giving the verbal cue first & backing up with the rein cue if/when needed. Pretty soon he'll respond to the first cue to avoid the pressure from the rein/leg. You can do the same sort of thing with anything new. Eg. if you want to ride him in a halter, I'd put a bridle over the halter, reins on each, but only use the bit reins to back up other cues.
In order to get him to halt I have to practically move in front of him, and even then he acts like he has just shut his brain off.
If he is so uneducated on the ground, then I would be starting at the very start there, teaching him basic yielding to begin with. If he's never been taught anything on the ground, the same cues from the saddle won't necessarily transfer either - horses aren't good at generalising. Setting up the situation to make the desireable behaviour as easy as possible for him to do. Eg. getting in front of him is a clear, 'easy' message that he must stop. The normal way however, of asking for a slow/stop on lead, which he very likely understands is to just put some backward pressure on the lead to get him to stop. Without knowing how you're goiing about it, can't really offer specifics.