• Would the impact knock him over? It's not like the movies. One gunshot, even with the caliber you're describing would not necessarily knock him off his feet. The pain and blood loss (depending on what injuries you ultimately pen in) may drop him, but the impact of the bullet itself? No - it wouldn't "knock him over."
• Would he feel pain immediately and how severe would it be? Large caliber weapon from fairly close range? Yep - pretty darn painful. The severity would depend on his adrenalin, pain threshold and what organs were hit.
• What kind of blood loss would there be? From the wound? Unless a large vessel was hit, there may not be much. Skin has an amazing ability to stretch as the bullet rips through, so it would [somewhat] close. Entry wounds are smaller - exit wounds are horrific. The entry wound would leak some blood out of the wound. Internally, depending on what you have get hit by the bullet [am I sounding repetitive?], your character could lose a fair amount of blood. If his aorta (main blood vessel carrying oxygen-rich blood from the heart to the rest of the body) is hit - he could bleed to death initernally within minutes. Other arteries (such as those that supply the colon or small intestine) could lead to death by blood loss internally unless caught and operated on. The range in endless, depending on what you "need" from the injury and time frame you're trying to create.
• Would he remain conscious? Depends on the blood loss you want/need. If you want him to, then he will. If you need him to go out.... so be it.
• What kind of internal damage might the victim suffer [it needs to be serious but non-fatal!] and where might the bullet lodge? If I read your description correctly, he was shot from the front and the bullet enters "to the right." His right? You're looking at possible small and/or large intestine damage, kidney damage, spinal damage if it tracks toward the midline... a .357 from that distance would put a serious hurt on him internally... especially if you have him shot with some of the more brutal types of ammunition (Black Talon, Hydrashock, hollow-points, etc.) It could lodge anywhere you want - spine (with or without nerve damage), kidney, muscles of the back... Is there a reason you need it to lodge in him? A serious, but non-lethal gunshot could be a through-and-through entering from the front, travelling across the abdomen and exiting from the back, but near the side. It could hit intestine and necessitate surgery to save him, but not kill him...
• Could there possibly be any collateral damage from bone/bullet fragments; scraps of fabric in the wound? [It’s an hour or so before his friends can get him to hospital] A scrap of fabric in his wound would be the least of his worries. Assuming you're getting him to a hospital and surgery, he's about to be cut open and have people put their (hopefully sterile) hands in him... a piece of his shirt won't be the death of him. The collateral damage would all depend on what you want hit and what you want the recovery time/course to be.
• How long might surgery take and what are the risks that his recovery might be hampered by infection? Depending on what was hit, surgery could take anywhere from an hour to several hours... does his spine get hit? Does he need a tedious, microscopic neurosurgery? Do you need to call in specialists? Is his colon injured to the point of needing a colostomy? Permanently? Temorarily? Will he need follow-up surgeries? Up to you. Infection is always a risk. The would would be flushed during surgery with antibiotic solution and a course of intravenous antibiotics administered during his stay in the hospital, but sure - there's always a chance of infection. It's up to you if you need/want him to have to fight off infection or not.
• Would there be any long-term health consequences [this guy is in the military so needs to be pretty fit to return to his line of work]. You got it... depends on what was hit. My "minor" gunshot (not that I want to be shot with a .357 myself, no matter how 'minor' you call it...) scenario could end up with exploratory surgery, repair of the bowel w/o colostomy, a hospital stay of a few days to a week or so and no real long-term consequences.
Huge thanks to anyone who can help… I have tried to get info online but it’s all really dense medical research! My suggestion? Try to interview a trauma surgeon or emergency room doctor. Fifteen minutes of their time will give you all you need (and more).