I personally am not a big fan of shaving cats in general unless it's an extreme situation (I know a longhaired cat who had gone all his life without grooming and was so severely matted the only way to get them off was to shave him, for example, and he's now being socialized to grooming while his coat is growing back out to prevent it from happening again) but I think in your cat's case a trim would be helpful. I have a half-Ragdoll girl who has that fluffy coat and while she's never gotten professional grooming I've given her trims here and there, because she does the same thing your boy seems to be doing with the neck ruff. Trimming hers down a little bit seems to have stopped it. I would suggest keeping him trimmed down to a manageable length and work on socializing him to brushing if he needs it to help keep up with the mats better. I just hate to see a gorgeous long coat get shaved when there's other ways of keeping it taken care of. A trim is cheaper and easier to get done, too, and if you give him some practice at home (trim little bits here and there, just a tiny bit off the end so it doesn't scare him or make him look patchy, and give him tons of treats and loves for letting you) he'll learn to sit nicely for a full trim at the groomer's and make it way easier for everyone.