Firstly, you don’t need to strictly follow the guidelines on a bag or can of puppy-food. These guidelines are meant to encapsulate the feeding requirements of male and female puppies, from hundreds of different puppy breeds, all with different metabolisms, at different ages of life, so the chances that these guidelines are specific to your particular puppy is pretty slim.
To find out how much your puppy needs, really it is about observation and taking note of how much your puppy is actually eating each day right now. You might want him to eat more because he’s a little thin, but if you don’t start noticing how much he’s actually eating right now, he will never eat more.
If your puppy looks healthy and is gaining weight with the amount of food he’s eating, then the amount he’s eating is the correct amount for him for now, regardless of what the instructions of the food package says. If your puppy looks thin, then he might need more, but you need to establish how much he’s actually eating in order to increase how much he eventually eats.
In a multi-dog household where there is more than one dog, then if a puppy didn’t eat all of his food, another dog-family member would eat it, so puppies learn to eat food when food is available and they seldom become food-fussy. When puppies are the only dog in the home and they notice that when they leave something in their bowl, that it is still there when they come back, and they were not that interested in it in the first place, then over time they become less and less interested in food.
So for that reason, food can only stay down in their bowls for 15 minutes and if they don’t eat, it needs to be removed. If it isn’t removed, they will start to lose food desire because it is always there so they will learn to only eat when starving, meaning their stomachs get smaller and smaller and over time they will start to eat less and less because food is not valuable to them at all.
So right now observe how much your puppy is eating and if this is as little as 100 grams of food per day for example but you want them to eventually eat 200 grams per day, then if you don’t stop over-feeding them right now, you will never increase their appetites and they will remain thin for ever (unless you get another dog).
So you need to start with the maximum about that they will eat per day right now and not a gram more, and build from there (if you need to).
Then using 100 grams of food as an example, this needs to get divided out over 3-4 meals spaced out over the day.
If you choose to feed 4 meals then say, then each meal needs to be only one quarter of their 100 gram allowance and it is essential to not give any extra in any particular meal. So for breakfast they would get 25 grams, and the same for each following meal. If they don’t eat any or at least all of their meal, then you do not give extra than one quarter if you feed 4 times a day or one third if you feed 3 times per day in any following meal. Again this is essential because you want them to want more at the end of each meal so they need to be licking a bowl clean if you want to increase food desire. So you need to stick to one quarter or one third of their food allowance (what they’re actually eating right now) per meal.
You also need to make their food tasty, especially for a fussy puppy, so if you’re feeding just dry food, then this needs to be mixed with a little wet food to make it more palatable. Do not mix in things that a puppy can pick out, so do not add pieces of meat, because the puppy will just learn to pick out the good bits and survive only on these. So if using meat, it needs to be pureed. If using wet and dry food, then wet food should be mashed up (even mixing in a spoonful of water if you need) and the dry food needs to be covered with this mash, so that tastier wet pieces can’t be picked off.
N.B. While tiny (I mean tiny, so half the size of a pea) training treats can and should be given for training tasks, you should not give other things like food from your own plate, at least not while you’re trying to teach them to eat their own food, no matter how cute they look while looking at you eating your toast.
When you eventually have your puppy eating well and if you then do need to increase the amount of food they get because they are becoming a little thin, you do this gradually over weeks, and following the one quarter per meal 4 times per day example above, you would add a little extra to each meal only when they are eating well. So after a couple of weeks they would be getting 30 grams per meal, then another couple of weeks, 35 grams per meal and continue like this building up slowly if you need to, until your puppy’s body looks appropriately healthy.