I would feed him based on vet recommended amounts. However, it's not the be all end all. You may have to adjust the amount of feed he's getting if he starts getting fatter or thinner. Right now, he sounds like he's at a very good weight.
Dogs eat less when they get older, as well. My two dogs are free fed. I feed them twice a day, but they usually have some sitting in at least one of the bowls at the next feeding. If I was a little less lazy, I might put them on a stricter schedule and it's important to watch that kind of thing, more so with two dogs than one, because you want to make sure one isn't eating his food, then chasing the other off their food.
If it bothers you, then you can put the food out for roughly 15min. After that time period, you take the food up. More than likely your dog won't eat much if any of it. The next feeding he'll mostly likely eat it all. If not, he'll definitely be hungry by the 3rd feeding, assuming of course he isn't being fed way too much or snacks in between. Once he's on a schedule, he'll know to eat when it's put in front of him. This is harder on you, but can be better on the dog. It gets him on a regular bathroom cycle, makes him more motivated for treats during training, etc... However, you have to feed him at those exact times and amounts every day, that includes going on vacation and making sure the person caring for him does it the same as well.
One other thing to keep in mind about recommendation on quantities is that the bag quantity is supposed to be based on the amount of nutrients in an average piece of kibble. If you dog is needs a certain amount of amino acids, fats, protein, etc..., then the bag is supposed to tell you based on his weight/size, how much to feed him to get these amounts in him. The reason people say this can be off and the reason you have to monitor it and consult a vet, is because this doesn't take into account a dogs every day life. For instance, a 90lb lab that lays around the house and goes to the river on weekends doesn't need as much protein as a 60lb GSP that's out working birds in the field every day.
Because of that, you can find foods that cater to specific needs. You can find performance foods that have more fat, protein, etc... in a 1lb of kibble than their maintenance food for a house pet. I hope this has helped point you in the right direction on his food issues. Like I said, from your description he seems to be just right. If he's leaving food and you don't want it sitting around, get a cheap scale and weigh his food before and after for a week. Find out roughly how much he's actually eating and then feed that, plus a tad more incase he needs the extra boost, but I wouldn't worry about him not eating as much.
