I've had my dog, a chow/lab mix, for a year and he's 3 years old. It's always been a problem that he doesn't eat very much, and as a result I can always see his ribs. I feed him around the same time every day, but sometimes I don't really need to because his bowl still has half the food from the previous day in it. I refill it regardless, and sometimes wait a few minutes to see if my presence will get him to eat... it seems he eats just enough to survive.

However, when my sister comes over and brings her three little dogs, he eats every morsel of his food and usually drags his bowl away from the others to eat it.

Is there an explanation for this?
How can I get him to eat more when the other dogs aren't present?

I would feed him at set times each day but only leave the bowl down for 10-15 minutes.
After that pick it up and he gets nothing until the next meal.
He will soon learn that if he doesn't eat it faster and doesn't eat it all then nothing else will be put on offer.

That's one of the problems I find with free-feeding, or leaving food out, the dog tends to pick at the food or doesn't stay and have an entire meal, they know it will be left down so why bother finishing it now?
It's a dogs instinct to leave food for later, that why they will bury bones or hide bisuits to save for a later date. You need to break that and get him eating set meals.