I have had a lot of experience with this.
My daughter had trouble with the triangle grips. We had much better success with the claw.
link to cheapest one I found on internet.
http://www.especialneeds.com/aba-and-au ... aptivaFeed
The underlying reason he is wrapping his thumb is due to fatigue. His hand is weak. We used a great fun and cheap exercise to build muscle in her hand.
Treat it as a game, in 1-3 minute sessions 1-2 times a day. Have him with his "handwriting hand" rip out a page and crumple it up into a ball and throw it into a trash can.
The other hand may not participate in helping. So make sure the phone book is nice and heavy. Play along with him see who can do more. I'd even reward him for every 1-3 he puts in the trash can. Getting a pile of M&M at the end will encourage him to do more and more and more.
other things like daily play with play dough, putting coins in a bank, holding a handful of beans and sorting them one by one into an egg carton all improve dexterity and hand eye co-ordination. I also think stuff like climbing trees and monkey bars help too, but that is purely my opinion.
Also as a fail safe "if you haven't already" don't forget to get him an eye exam, to rule out any eye issues.
I also would go for less work
but done really well. My DD was only required to do 5 of the letter, really reduce his work load so He can do it fabulously without fatigue as you grow his hand strength. It took us 6 months to see improvement so don't stress if it takes a while.