I personally prefer packaged food as it is very convenient to feed and has all the nutrients in the right proportion. For Labs, I think there is a tailor made food by Royal Canin. Royal Canin as per me is the best and the Eukanoba. You will need to see how much you want to spend and then accordingly choose one of the above or others which may be a bit cheaper. A new food called Natural is also out which is good. Most dog food will have a guide as to how much to feed your dog based on its age, breed etc.
I hope u do not feed ur dog in between snacks etc. If it does not finish its food within half an hour remove it. By giving just one type of food the dog will not become a picky eater and will eat its food when given, if it sees that u will remove it after a while.
Loss of appetite can also be due to worms and I prefer deworming my dogs every 3 months.
All The best !! Hope I was of help.