I got a 5 month-old guinea pig off Craigslist from a family who could no longer keep him a week ago. He looks healthy. Has clear eyes, acts normal, and doesn't feel skinny. But, he is 584 grams. I just weighed him today and I read adult guinea pigs should weigh 900-1200 grams for males. Should I be concerned and set up a vet appointment?

My vet does a free checkup for new pet, but I am scared if I bring him, he will get scared and stop eating and drinking.