My cat seems to have a scratch below his neck, but he is really averse to showing it to me. He seems fine, but at certain angles the wound looks rather big while in other angles his fur covers it entirely. My question is how can I get him to let me look at the wound, so that I can understand how severe it is?
In my experience the easiest way is to be sneaky about it and peek at the wound while the cat is eiher relaxed or sleeping. The best way to achieve that is by petting. You may have to invest some time to really get them to relax and lay down.
You also may have to try several times. If your cat lays down in a way you cannot see the wound and you try to move it around, the cat is probably going to be suspicious and walk away.
A small hand mirror may come in handy to really look at the wound from all angles without making your cat suspicious. You may try shining a flashlight at it to see better, but again, that might make your cat suspicious of the object and walk away.
As well as petting your cat to get him to lie down, use a clean wash cloth or rag that is dampened with warm water when you pet him. It should be about as damp and warm as being groomed by a cat tongue. Cats naturally lick wounds so this will feel more comfortable and dampening the fur around the wound slightly will help keep fur flat and make the wound more visible.
Your cat is not going to let you "study his wound". He's a cat and it's going to be difficult to accomplish such a task if he won't let you already.
What I suggest is taking a video using a good device like a 4K camera or smartphone. It'll give you the few seconds of time looking at the wound that you need to get an in focus shot of the injury without causing too much stress to your cat. Remember videos are just images taken in a sequence and each second of video is typically 24-60 frames of slightly unique images.
Most of the frames are going to be in focus if the video looks like it's properly focused so you end getting a good amount of usable different shots. Better video camera usually have various settings that can increase your chances of getting useful footage quickly, like ISO speed settings. I found this useful to look at my cat's teeth after I accepted the fact that he's just never going to say "ahh".