Optimum Nutrition makes the best weight gainer IMO. If your willing to use weight gainer I would recommend this one:
Bodybuilding.com - Optimum Pro Complex Gainer - Over 600 Calories! On sale now!
only 6 grams of simple carbs out of 85 and 60 grams of protein. 850 calories. One shake in the morning one at night and you should see improvement in a week or two. I would at least recommend some decent resistance training though.
BTW I'm not advertising that particular product for my own benefit. I have tried it out. No stomach upset. Many gainers have large amounts of sugars.