Building muscle

Gradually up your calorie intake for a couple of weeks and see if you start gaining weight. The key to minimizing fat gain is to keep the weight gain to a minimum and train hard. If you are gaining about 1 pound per week, you're good. You will gain a little bit of fat, but as long as you are training hard and eating right, you can diet it off later. Watch the mirror, and if you start looking too fat or flabby, then it's time to cut. Just don't go overboard and gain a lot of weight fast, then try to diet it off. Look into Layne Norton's PHAT program. It's a good body builder program.

From what I recall when I was putting on muscle mass from working out in college, the practice was to consume a minimum of 1 gram of protein per pound that you weighed. Therefore, you would need to consume somewhere in the range of 170-190 grams of protein for lean muscle gains. That is a lot of chicken and eggs to eat, but that is why there are protein powders and pills to supplement your meals with. Try a protein powder called ISOPURE, high in protein but very low carbs, taste is good as well. It is in the range of 50 grams of protein and 200 Calories a serving. Hopefully this helps.

If he's going to avoid carbs he is going to have a really hard time putting on weight. 3000 to 3300 cals per day is a good place to start for the OP, assuming his strength training is sufficient to spur muscle growth. and 140-170 grams or protein would be the general area I would shoot for if I was him.

How much weight are we talking about?

A half pound to a pound a week is a good pace. 6 months is a good duration.