Has he been neutered? Some young 'entire' male dogs can burn up a lot of energy, and have a problem gaining weight, even though fit and well. (effect of all that testosterone) I knew a male dog like that. He was so skinny, though very well. It was like his energy was being burned off him. When he was neutered he was still as fit and active but seemed to fill out a bit more.
Of course it could be that he needs a higher calorie food to cope with all the activity and exercise he gets. A more athletic dog needs more substantial food. He may not be getting enough support from his food, though he does eat regularly.
Brown rice is a good carbohydrate source for dogs, so is pasta mixed with his food. Potatoes are ok for dogs too. But it might be best not to give him a food which is high in saturated fat. Go for good protein, like beef, chicken, venison, rabbit, sardines, tuna, and omega oils, and carbs.
Because I live in England, we have different dog foods over here, so I can't recommend a food for you to try. But with a little research, maybe you could find a suitable food.
If after all, he continues to lose weight, or really has a problem gaining even a little, maybe it would be a good idea to take him to be checked at the vet.
I agree with ginger that your dog needs a vet check. If you can see ribs through a poodle's coat, something is wrong. It could be anything from poor quality food, to worms or something really off the wall. There's no point in speculating, so it's time to get to the underlying cause and get professional treatment advice. Let us know what your vet says. :-)
You may have one of the few dogs that do well on a high protein diet.
Orijen, Evo, Wellness Core, or Eagle Pack Power formulas may work well for you. Also deworming may help since failure to gain weight can be a sign of parasites.
Another alternative is to use puppy food of the brand you are using. But make sure it is a high quality food with a very digestible protein (chicken being the best).