My son does almost the same thing. I have to be honest with you, I just gave up correcting him. I think it's probably what's most comfortable for him and it probably has to do with the difficulties our kids have with fine motor skills (his handwriting is horrible too). I finally just figured that as long as he is eating and the rest of his table manners are good, I'm good.

Shorter utensil or pencil is perfect for teaching proper way to hold pencil or fork...I forgot that we did that for school beginning last year. Worked great

It's pretty much ingrained after the age of 6 and most adapted utensils would only encourage this grip even more. If his grip really bothers you you could try offering a shorter utensil as it's harder to hold a shorter utensil with a fisted grip and you get more stability with a proper grip so he would be forced to use it.

My son ate same way for a long time. I just kept showing him the correct way to hold a fork...He's 12 now and holds fork correctly. We have worked on cutting meat...that is now getting better ; ). He also holds a pencil the same way. Was never corrected in school but he can hold the pencil correctly, I started homeschooling last year and just have to keep redirecting him. It's constant re-directing without fussing or nagging at him. Takes a lot of prayer : ) but he can do it.