First he needs to fix his diet, and secondly he needs to get into a basic exercise program.
This would be a good diet to start with as a sample. He basically needs around 2,500 cals per day, with at least 200g of protein.
2,500 cals (approx)
Meal 1 - 3 whole eggs + 5 egg whites + green peppers and onions + 3/4 cup oatmeal (uncooked volume) + handful of mixed berries (any)
Meal 2 - 6 oz grilled chicken + 1 cup cooked brown rice + 1 large salad + 1 tbsp olive oil + vinegar
Meal 3 - 2 slices whole grain bread + 1 can of tuna + 1 oz of 2% cheese + 1tbsp mayo + veggies (tuna sandwich)
Meal 4 - 6 oz grilled sirloin steak 90/10 or leaner (trim off the fat) + 1/2 cup black beans + 1/2 cup brown rice + veggies + 1 tbsp olive oil
Meal 5 - 40g protein shake + handful mixed berries + 1 slice whole grain bread w/ 1 tbsp peanut butter and 1 tsp jelly
Meal 6 - 3/4 cup of 2% cottage cheese + 1tsp heavy cream + handful mixed berries + 10 almonds + cinnamon to taste
As far as exercising goes, he needs to start with this:
http://forums.d2jsp.org/topic.php?t=21269614&f=60&p=331731288Bodyweight exercises
And eventually he will need to get a gym membership or home equipment.
Also a good abs routine::
http://forums.d2jsp.org/topic.php?t=21269614&f=60&p=330745031Abs Workout Routine
2500 cals is gonna be a bulk for him, if his main concern ATM is abs would they bulk just not kill his ab plan?
I would think he has to cut if he wants to get abs but he's almost at underweight based on bmi calculators.
But then again I'd prefer he bulks cuz he is so skinny.