If he isn't getting the idea via you lifting his paw for him - how about trying a slightly different method? If you change your approach, chances are, he'll change his response.
An approach you might try is, instead of lifting his paw, just give your command and then tap him lightly (but annoyingly LOL) on the shoulder. He's sitting facing you for this, btw. The usual response from a dog to that sort of irritating tapping on the shoulder/top of leg, is to lift that leg. You then take his lifted paw lightly in your hand and
immediately praise and treat. Then as soon as he's scoffed his treat, let go (don't hold his paw long enough that he thinks of boxing).
It's not enormously different from what you've been doing, but it is different in a few important ways. First, it's not an approach that he's got a pre-determined response to (the boxing). Second, the paw is lifted under his own steam - not yours, so there's nothing for him to react to (less likely to box). And third, you aren't handling his paw for long - and are praising/treating him during the time you're holding the paw. Once again, he's less likely to respond with a box, because he's too busy getting his treat. All of that just gives him time to learn the command and what is going to happen (you're going to hold the paw
he lifted, give him a yummy treat, then let go) before he thinks of a response. And having discovered that, is less likely to make any response anyway. It's worth a try

As ever with dogs, if what you're doing isn't working, the dog just doesn't "get it" - try something else.