Yes, please be very careful about having the right papers - if you do not have your ducks in order you may be denied your cruise with no refund or compensation from the cruise line.
This is just one of those very sticky situations where the cruise line cannot allow itself to be in a position of taking a minor child anyplace a legal guardian does not want him to go.
Now - even if no one can get a hold of his dad, that does not mean the cruise line will let it slide. You need to have either full legal guardianship or else get the dad to sign a release saying he can go on the cruise.
They won't get involved in your personal details or life - its all about the paperwork.