That must be so difficult. I can't imagine losing one parent, much less both so close together.
I agree, do what you normally would for his birthday. Don't assume you shouldn't just because of this. Especially since he is a private guy like you stated. If you didn't do the norm it might draw more attention to the other issue and he doesn't need that either.
And I also agree with what others said, just be there for him. However that may be.