Constantly runny poo definitely doesn't sound right. Is he going frequently during the day, or does he have a predictable poo schedule? And is he otherwise fit and healthy?
Which foods have you tried? Labradors do sometimes have problems digesting wheat or rice. Burns do a potato based one which has certainly solved a similar problem with both a lab and a spaniel I know of.
Just a thought though - is he a hoover up everything sort of dog? Is it possible he has found a regular source of something vile? The vile things they just love sometimes just don't love their digestive systems.
A nightmare to sort out and pick up after! Been there myself. Has he always had runny poo? I would take him to the vet just to check out parasites, and camplyobacter and worms. Usually a blood test. When you get that back clear hopfully then you can look at diet.
More and more dogs are being allergic to cereals so you may want to try a potato based food. Sometimes feed just chicken and rice home cooked and then add food and it is easy to see the culprit. This can take a while to sort out. Also as alreay mentioned over feeding will cause runny poos. So the thinner the dog gets the more you feed and the thinner the dog gets! So give less food and see how you get on. I would stick to one of the better quality food at the moment Burns, Arden Grange, James Wellbeloved ( a bit of a marmite food) and if really wealthy orijeen!