Have him keep track of his calories with something like fitday.com. I'm in the same situation - unintentionally lost 20 lbs while eating paleo-ish, and now I'm trying to put some of the weight back on.

Tracking calories will remove some of the guesswork and help him to make sure he's eating enough. I don't get super precise about it - I put in the main ingredients of what I eat, and guesstimate portions a little low so I'm erring on the side of eating more.

If he does dairy, you can consider having him drink a couple big glasses of whole milk per day. It's an easy way to get in some calories, and has helped me a lot since I don't have that big of an appetite to begin with.

It's the same as losing weight, it comes down to calories. If he wants to bulk up, eat clean, but unfortunately he will have to eat more. I know you want something better than that, but that is the truth.

He doesn't necessarily have to up the carbs. If he just sticks to strength training, and gets rid of the cardio, he won't be depleting his glycogen stores enough to justify a big increase in carbs.

I would carbs if he feels he is not recovering fast enough, or doing more intense cardio... which if you are trying to bulk up may not be the best idea.

Keep the diet Paleo clean.. for easy way to up calories... coconut milk/butter, nuts, and starchy veggies, etc.

Just eating more protein wont do much unless you weren't getting enough to begin with. Otherwise you body might just this protein to obtain more glucose. Up the fat or carbs.