Also, where abouts in the world are you? Because the correct way to hold a fork is a regional thing, so it will be easier to judge how bad an offence the 'shovelling' is if we know what country/area we're dealing with
I would suggest, for starters, having him over for dinner and pointing out (gently) a few of the problems.
He starts to 'guard' his food with his elbow? Maybe you could jokingly ask him if he's worried people will steal it
Otherwise, depending on your relationship
, sitting down for a heart to heart could be the way to go.
"Nephew, you're a professional lawyer (or whatever) now, and we're really proud of you. You're likely to have a lot of business lunches and dinners in your new career, and I know this sounds silly, but I'm worried about your table manners. You have a habit of waving the fork around over your head when you eat and looking like you're guarding your plate. I do realise it doesn't reflect your work, but some people really notice and care about such things. It doesn't look very professional, and we want people to think the best of you. "