Is he skinny, or is he just small? If he's small, that's his genetic lot and there's not much you can do about it. If he's skinny -- more than a rib or two showing, hip bones, spine points, etc -- have you had a veterinarian examine him for internal parasites? They would need to run a fecal and/or blood test to rule out any yuckies inside him.
If there is no health reason for his skinny-ness, switching to a better food might be a step in the right direction. But not Diamond, IMO. First thing you should do is pick a high-quality food that you can afford to feed long-term. If you buy your food from Petsmart, probably the best thing they carry is their Blue Buffalo formula. There are other brands which are higher-quality, too, such as Taste of the Wild, Nature's Variety, Solid Gold, etc. Those you will need to go to a dealer or specialty pet food store for. As a rule, anything you see carried in the grocery store is the lowest of the low. Pet superstores carry stuff that is a bit better, and specialty retailers carry the best.
Here are a few resources to help you decide on a good food.
Dog Food Analysis - Reviews of kibble
RATE YOUR DOG'S FOOD
Whatever you pick, you should slowly transition him over and not switch cold turkey. He will need time to internally adjust to a high-grade kibble. Of course, that will only help if he will eat it. Establishing good habits helps. If you free-feed -- ie, leave the food down for him to eat at his leisure -- stop. Put the bowl down for 5 minutes, then take up what he hasn't eaten. If he is easily distracted, I would suggest putting him in his crate or a closed-off room to eat, so he can focus on the task at hand. You can use a bit of canned food, water, or oil (olive oil, flax seed oil, fish oil, etc) to entice him to eat at first, but you don't want him to get in the habit of "holding out" for better stuff if he's bored with his food.
As a rule, a healthy dog wont starve to death if there is food available. Exercise patience and consistency. He should come around with time.
PS, I saw the other thread and he didn't look skinny at all. Maybe it was just hard to tell from pics? Or maybe you're accustomed to seeing a bulkier dog? Can you post more pics of him?