I am also wondering if you mean 8c total or 8c each. Just based on the fact my EM puppy currently eats 8c on his own, he weighs ~150 lbs, I would be surprised if your doing 8c each. If you want to continue free feeding, which does make it harder to monitor how much each dog is eating, I would increase the amount left out. Unless there is a medical issue that either hasn't been mentioned or is still undiagnosed he's not getting enough food.
Personally I would stop free feeding because it is not working for ALL of them. I would take the inconvenience of meal times for the increased knowledge that each dog is getting enough food. I do have experience with this in cats. I have always free fed until I got Skreea. I would put down the correct amount for 2 cats and discovered that she was becoming overweight while the other cat started losing weight. By going to meal times I was able to control how much Skreea ate so that both cats got the correct amount.