I have a 6 month old lab that does not eat like he should be. He seems to be small for his age when I compared his to other labs. His breader (danferlabs) who is said to "be the best lab breeder in mexico by pedigree mag" said that I should only feed him once a day and give him as much as he wants for 10-15 min and then take the food away. I tried that and he would only nibble at it and move back to his chewing bone, and then the next day I assumed he would be starving for food but did the same thing for the whole week. Finally, I went back to just leaving his food out and he still only picks at it and will only eat maybe 2 cups of food when 4-5 is the recommended. I trust the breeder but is there anyway that I can get him to eat all his food and all of it! I have my lab on Nupec which is "supposed" to be one of the best for labs, but I just want him to be big and strong! Any advice

Try mixing something through it. I use either a little wet food or a bit of tinned sardine.
I also feed 2 meals a day. One around lunch time, and then one later at night. If I feed at breakfast and then dinner he doesn't seem as interested.

A nice big walk in the morning can stimulate his appetite, but just make sure you wait an hour before you give him any food.

Mix about 1/2 can of good quality canned dog food (I use Iams) into about 1 1/2 cups of dry dog food and feed him twice a day.

Once he starts eating it all, you can increase the amounts of canned and dry food until he's up to the max for his weight.