My biggest concern is his age at the moment, a bigger rider should never ever be riding a young horse, they need to be at their full strength before being asked to carry the heavier loads.
Next it is a question that I have been asking for a while, everyone says that Arabs are good weight carriers, being short of back and good solid bone, but then they keep quoting the same 20% as everyone quotes for any other breed, which I find very confusing.
Now a couple of people who have only seen her online have met her recently and have been shocked how solid she look when you actually meet her. I'm losing weight at the moment with the aim of being able to ride her, and my first target is 250 pounds, at that I would get on her and walk her around, but I would like to get to 225 before I ramp it up a bit, but that will all depend on how she does.
That's why I said to give him time over the next couple of years to build up to carrying a heavier load. The OP is not riding the 3 y.o., and is having a trainer ride him and train him. If the trainer is worth his salt, he ought to be light enough in the saddle that the horse doesn't suffer any damage from carrying him.
I don't know why people go for the 20% thing, I suppose that's optimum but I also think people are getting carried away with it. I think it was originally deemed for horses in hard work. Your average pleasure horse, especially here in the US, might get ridden 2 or 3 hours per week IF their owner is really active with them. I know many who don't ride except maybe an hour on a weekend, 2 if they go on a trail ride and that's mostly at a walk, maybe a little trotting tossed in for good measure. It's certainly not what I consider a work out.
I think there are differences for differences in ability too. I know people who weigh 150 lbs but have no muscle tone that I would not allow on my Arab's back. But his trainer was 6'9" and weighed 300 lbs and he never missed a step. His trainer could have been a quarterback but instead had one of the lightest seats and gentlest hands I've ever seen.
I don't like blanket statements and assumptions made over the internet when people don't know the horse or the rider involved. Too many things get said that can be hurtful and discouraging, as is demonstrated by the fact that the OP's first trainer was an oaf and now she's still questioning whether she should ride her own horse. That's not constructive to anyone.