My 15 year old MC gets shot in the stomach.
Wrong. You need your MC to be wounded by a firearm, in a way that is survivable. Now, work backward to figure out how and where he was shot.
What short-term result do you need? What long-term result? Base your injury on those needs, not a preconceived notion that he's shot in the stomach.
So, for a short-term requirement of emergency surgery, and assuming no long-term effects, an artery can work. Though you'll need a trained surgeon or responder to stitch/close the artery, a field surgery can work.
The vast majority of gunshot wounds occur in battle, and no battle wounds normally get stitched or have surgery when professional help is available within minutes, or even hours. A femoral artery could get clamped or tied off though, and sutures could work to tie off the artery for future treatment and life-saving.
Any normal gunshot wound to a non-threatening location, outer thigh, buttocks, bicep, calf, etc. could simply be stitched until future medical help is available. A simple dressing would be better, but whatever works for your needs might be acceptable.
Jeff