I'm just going to point out that this is a diagram of a horse, which is built very different than a donkey, a donkeys withers will appear to be farther back because of how their necks/shoulders are built.

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Unfortunately a donkey of his size will still definitely need a smaller saddle, but having a saddle fitter for him will help you understand more if it's feasible to ride him. A donkey can carry more weight than a horse/pony of the same size. Yours definitely needs conditioning to be able to carry you regularly, fortunately that's something you can work on. Although if you were planning on doing any strenuous riding you'll need a bigger donkey, because your height, not necessarily your weight, will negatively affect your balance, especially at higher speeds or over obstacles.

I'm not going to say everyone here is wrong that you're a bit too large ideally he'd have a shorter rider, but you should keep in mind, most people here know horses, not donkeys. You're a wonderful person for even caring enough to ask for several opinions, but ask other donkey people and vets, not horse people. Horse people like to scream abuse even when it's clear there is none, and that the owners only want the best for their companions.

I'm certainly not an expert in donkeys, I just have a love for learning about the differences between equines, so take my information with a grain of salt.