Looks like a pretty normal horse to me. Quite short in the back so the length of the saddle MAY be a factor but this horse is a pretty normal shape really. Average wither, about average curve to the back.
I have a lot of trouble because I have a horse with a straight back (LONG straight back thank goodness or I'd be REALLY struggling), he is wide but has a medium-sized wither so most of the wider, straighter saddles don't have enough wither clearance for him.
I really can't help with suggestions of what would fit as I only know what fits the shape I'm trying to fit!