There are consequences. He no longer gets to vote. He no longer has access to US-funded social programs. There is a long list of things he can no longer enjoy as a US citizen; natural consequences of not being a citizen. It sounds like you support introducing unnatural consequences.

I once visited the US on business and left without purchasing anything more than a meal. If every action must have consequences, what consequences would you suggest for my actions? Perhaps I should be banned from re-entering the US since I didn't contribute significantly enough to the US economy while I was there?