I have a 4yo gelding that I noticed being pretty tender under his belly, from about the back rib to his sheath area and on both sides (on the underside, no soreness on his sides). At first he tolerated me feeling around, but once I put more pressure in the areas he gave a jump and kick on both sides.

Any idea on what would cause this? My first thought was ulcers, but he has never been cinchy, or had any issue with my legs on his sides. Are they sore that far back? I always thought they would get sore more up front?

He is on pasture 24/7 with access to grass hay. His coat is nice and shiny, he looks good, tends to be a harder keeper, but his mom was too.. He has always been cold backed, but will ride out after 10-15min. He has always been super sensitive, but I only noticed this spot after he’s had about 3 weeks off due to the weather. He’s never flinched before in this area when I was riding him 3-4 times a week and I know I would have noticed it.

 Any input is appreciated

In my limited experience that's where they show ulcers. There is a pressure point in the girth area that supposedly shows ulcers too.

2 finger widths in either side of the navel are acupuncture points for parasites. I would make sure he's been wormed with a good wormer lately.

Has he been tested for PSSM? Hard keeper but look good, noticed it after time off, super sensitive... all signs I have personally experienced with one that had it. 

He is on a good rotational worming schedule; I'll get a fecal done to see if there's anything showing up. 

Hes not been tested for PSSM, and at this point I don't think he fits, but will keep that in my mind. 

I watched the video for ulcer acupuncture points and he reacts/Looks back at me for all the spots, but not sure if he's just watching me or actually reacting.. the only spots he flinched is far back under his belly and a little on his lower back.