Ok - I've read your request, and I've read the responses thus far. And, I'm talking to you as a former thumb sucker; leave him alone. That sounds rude - I know. But trust me, that's the best way to get him to stop.
I sucked my thumb until I was 9 - I didn't have problems with my teeth; my mother sucked her thumb until she was 12 - she didn't have teeth problems. Please, please understand that I'm NOT saying that nothing will happen to his teeth. I'm saying that sucking his thumb is not a promise of teeth issues. It IS possible, but not a given.
If you are still concerned, then here is my suggestion: tell him that he CAN suck his thumb - in his bed. Not in his room; not just out of sight - it HAS to be in his bed. This is not guaranteed to keep him from doing it. But, it will take the pressure off of him. He can suck his thumb, but within certain perameters. But, you'll have to spend time reminding him at the start. "Remember, you're supposed to be in your bed if you want to suck your thumb. So, would you like to take your thumb out of your mouth, or would you like to get in your bed."
My mother had a thumb guard when she was a kid. She said that every morning, she had her thumb out of it and was sucking her thumb. So, that kinda seems like a waste of money to me.
He will stop, but if you're reminding him a lot, well - it's a control issue. The thumb sucking is something HE can control and you're trying to take that away from him - kinda scary!(for him)
My oldest (almost 7) has an issue with clearing her throat. Every 10 seconds or so she does it. She has been doing it for a long time. Asking her about it causes it to happen more. So, I know what you're going thru - I know the frustration. But reminding him about it, well it will only make things worse.