I believe he is also standing a little twisted in the picture that may be some of it.
I am not seeing him standing twisted....
From poll to top of tail he is straight and nearly square with just a touch of his hind in front of the other looking from tail forward.
When I look at your horse really critically I would estimate he needs a solid 100++ pounds...
His actual wither bone is really prominent, his shoulder has no excess softness to it, he does not softly blend neck to shoulder, he has a missing fat pad at the wither, his hips are prominently seen, ...flanks are very concave tells he is very underweight although he still has from your picture angle a filled butt...but not a filled topline...
Pictures can and often do tell a real story but can also lie a bit to us too...which this is is unknown to us, but...I would not be trying to buy a
good/great fit with a animal in this condition.
This picture you shared indeed tells a very different story of this horses condition.
I would not be looking to fit a saddle to a horses frame that is this thin cause when he gains, any saddle you have fitting now will not. After opening your picture to full size my guesstimate of weight gain needed is upped to 200 pounds...no saddle is going to easily accommodate that change.
I also see where he actually has some decent shoulder muscling and again, thought his hind picture showed a bit of delineation, so although he is older and pretty thin, he is not without muscling like some couch potato animals are.
When fully weighted...this is no "narrow" treed saddle fits, never!
Even with his current condition
I do not see this horse as a narrow treed saddle and gullet.
If this horse was mine, he would have little exercise while I worked to get pounds on his frame...every calorie used to create fat on the frame and only after a significant gain would I start riding to build muscle...
When so much of the ribcage is seen, 3/4 of it from spine to belly the ribs are seen and this is with a decently started winter coat seen this horse is needing to concentrate his energies on gaining weight not being ridden in my opinion.
Give the animal 2 - 3 weeks of nothing but time to eat and gain, then start to look for something to fit him that will need some extra padding currently as he has a lot to fill in..
With winter coming by evidence of his coming winter coat, my concentration would be to get fat on his frame and only once he has gained a substantial amount would I start making him burn off calories by riding him and trying to build more muscle.
Personally, I am not a fan of changeable "gullet" saddles cause darn but the only thing that changes is the saddle head, not the gullet which is
not on a hinged joint so quite a misleading of "it can be changed to fit...".
Since there is no industry wide sizing regulations every manufacturer does their own thing so "I'll get this size in this brand, that brand and then another brand" to me is worthless.
Even from the same manufacturer there are differences with each model made from the factory forget if someone can flock the saddle underneath to fit the animals back that moment in time.
You are going to have to try each saddle as a possibility...over the trial and error of saddles fitting or not you will begin to see what has a chance and what to leave on the shelf and keep searching.
I wish you much luck in your future journey with this animal... he will be a animal you not recognize once he is fully weighted...enjoy the journey or returning him to good health and weight.

🐴 .......
jmo..