How much do you feed?

I current have a lil guy who turned 1 a week ago, I’ve taken him into the vet to get accessed, nothing wrong stool wise, but underweight and doesn’t gain weight easily.

Vet recommended 600 k/cal a day hoping to get him up to 35 lbs before switching off of puppy food.

Before vet recommendation / current I’ve been feeding 800 - 1000 kcal a day. He is still super slender and only 25 pounds. He’s been this weight for much of his life. Given he was 20 lbs by the time he was 4 months old.

Anybody else struggling with weight gain?