Absolutely agree with Cherie.
In this case, I'd be looking at the simple explanations before going with ulcers.
When was the last time you had your saddle professionally assessed and fitted?
How fast do you do the girth up?
Have you pinched skin in the past? - I know a school horse who was always great to girth, then one day a kid pulled the girth up too quickly, pinched some skin, and even since then he throws himself on the ground each time someone goes to do his girth up.
When was the last time you cleaned your girth? - is it rubbing?
What type of girth do you have? Some horses have a strong dislike to particular girths.
When was the last time you cleaned your saddle pad? A prickle jabbing into their back is a great excuse to be girthy!
When you get on, do you get on from the ground of off a block? If you're getting on from the ground without someone holding the oposing stirrup, you will drag the saddle over the horse's spine, and all of those muscles can get very sore after a short period of time.
When was the last time the dentist looked at the horse's teeth? Girthyness can be a symptom of sore teeth
How balanced are you in the saddle? If you bounce or slam down on his back, its little wonder that he's girthy.
Lots of simple explanations, I'd rule those out first before going to ulcers unless he's exhibiting other clear symptoms of ulcers.