I seriously don't see any problem here. I think the INTJ brother needs to tell the brother HIMSELF, not you (or maybe both of you guys should have talk with him). From my ENFJ POV, I would feel like it's none of your business and that you are trying to change a fairly neutral behavior. Unless, the ENFJ is acting like a jerk to his brother, there's no need to get involved.
Some people are just not very affectionate and that's just a part of life. I'm an ENFJ and I'm not all that affectionate... it's just who I am. Don't force him to change something like that. If someone tried to make me change something like that in my personality, I would be greatly offended. It kind of like saying "oh we don't like this part of you and we want you to change it make your brother happy. It would feel like you are JUDGING me. I am guessing the ENFJ shows his love through other means.
But like I said, if he's being a jerk to his brother, that's another story.
EDIT: You might want to read up on "love languages".