It isn't just about not overfacing yourself - you don't want to overface this horse.
Going with what you've already been suggested -
The horse has a weak topline and needs allot of work on that back end - jumping is dressage with speed bumps.
The thing with big horses such as this fellow, is that - they are allot tougher to manouver, allot more work to direct via seat, allot of time to tighten him up when riding through a tight course. Most courses at lower levels, are small and tight.
If his rider doesn't have the power and the know how - big horses can be allot more work. Trust me, I used to ride a 17.2hh Zangershiede - talk about allot of power, talk about allot of horse, talk about allot of animal to work under me. I had to learn to use allot of seat, allot of core and it took allot of time under saddle on the flat to reach the goal of having a light, manouverable horse under me - so that we could take that and apply it to a course successfully.
Big horses, are allot tougher to manouver through a course than a smaller, lighter horse.
This horse should be fine doing low levels - but he will need ALLOT of flat work under an educated rider, so that he can remain on his hind end, light and off of his forehand - to transfer that to a course, to remain light, soft and on his hind end. He needs a rider who can ride from his back end, leaving his front end alone - especially his face.
A horse this size, cannot be flat and heavy on his forehand. A horse this size needs a rider who can do this.
He needs a rider who can support him constantly, to the fence, over the fence and after the fence - keeping his back lifted and light.
Educated Flat work, Educated flat work, Educated flat work
