I haven't really researched why follow through is more important in smallbore than high power, but the velocity seems like a reasonable explanation. I have been shooting smallbore my whole life, so I didn't really notice anything until going to high power and finding it so easy. My first time shooting service rifle, I had 95 sitting, 93 standing, 95 prone rapid, 94 prone slow. I'm not a good standing shooter in smallbore, averaging in the low 90s.
Back to the issue at hand, I will admit that I am in charge of the workouts my college team do. They lift weights 2 hours per week and have 1 hour of cardio. BUT, we're looking to get maybe a few points out of that, and it's more of a mental advantage than a physical. The shooters have to learn a proper position in order to succeed in standing. And the first step is mental: learning to LOVE standing. The easiest position to shoot is the one you're shooting. Most junior shooters just starting out at best dislike standing, at worse are afraid of it.
To start from the beginning (for a righty), feet shoulder width apart with the target on the shooter's left. The heel should line up to point at the target. Make sure the offhand stand is close enough to be reached without bending anything but the arm. Spotting scope close enough to look through with only a slight tilt of the head.
Make sure the rifle's butt plate is all the way down, or in sporter the rifle is high enough in the shoulder to allow him to look through the sights without tilting his head too much. The line between his ears is the basis for his balance. If his head is too tilted, he will start to sway. He'll likely need a palm rest on the rifle (not in sporter obviously). He should support the rifle with his left fist with his palm facing in towards him. Since he's somewhat tall, and a guy, he likely won't be able to put his left elbow on his left hip. Therefore he'll need to support the rifle with his left upper arm on the side of his rib cage. He will be tempted to twist his upper body to the left, but he needs to keep his shoulders in line with his hips, and keep his whole body upright. If he doesn't have a hook butt plate, he'll need to apply a little pressure with his right hand to keep the rifle in his shoulder. This should be SLIGHT to FIRM pressure, not a death grip. If all of this is done right, he should be able to stand in position with little muscle tension for as long as he wants.
Obviously you can't stand without some muscle, but the more muscle tension you can eliminate, the better. This goes for all positions. He'll want to squeeze the rifle to steady it, but that will only make things worse. He has to relax and learn to like standing.