If choosing to believe in Santa means he continues to get an extra present every Christmas, and revealing his lack of faith means he doesn't, then it makes sense for him to choose to continue to believe in Santa for as long as possible.

But that's exactly why I said it's not possible. Some people may be able to forcefully change themselves into truly believing whatever is the most advantageous for them, but most aren't. Once the illusion is shattered, you usually can't put it back together because they can't just choose to just swap their system of reasoning - it will feel like lying and fooling themselves. Any atheist can pretend to be religious in a hostile society like how the kid can pretend to never having seen the parents, but it will just be an appearance that's not indicative of what they truly think inside.