Maybe tell him what you told us? Ask him if he's alright, because you've noticed a change in behavior which has made you concerned.

Perhaps there's nothing wrong, or he just wont open up. But he might appreciate your concern.

As for spending time with your dad. Maybe you could ask him what he wants to do. Though you might not like what hr chooses. If that matter sto you, another option could be that both of you could try something new that you're both curious about.