Hi and welcome to YT! Also, welcome to my world
Georgie was 1lb when we brought him home at 10 weeks. He is 7 now and his heaviest (funny term for him) has been 4lbs. Georgie would get a dime size amount of Nutrical on the tip of our finger twice a day. I don't remember when we stopped giving it to him I think it was when he stopped liking it. It was really like having a baby. My husband and I rotated coming home throughout the day and he was never home alone for more then 2 hours. I never wanted to risk hypoglycemia. Boiled chicken became our best friend. When he wouldn't eat his kibble he couldn't resist the boiled chicken. He still eats boiled chicken and still loves it
I have a few of pieces of advice:
#1 Any signs of lethargy take him to the Vet please do not wait.
#2 Once you feel comfortable please switch him to a better quality grain free food.
#3 Leave food out for him at all times. Once he has reached his adult weight and is not a puppy you can have him on a schedule.
If you need any help/advice I am here I know exactly how you feel with worrying.