Lifting the shirt isn't an offence, taking it off or having it cover your face is, from what I remember, he held his shirt at his neck, so didn't cover his face. (Though, I like to think the ref would have let it go in that instance if he had covered his face).
There used to be an issue on the nature of messages on shirts (weren't allowed to be political, social or religious comments, or any likely to incite), but that specific aspect of the rule seems to be missing.
Brave man for playing so soon after the death of his son. Your little boy will be looking down from heaven and be very proud of you, Billy. As the thread says it puts everything else into perspective at the moment.