baby's weight

How much weight has your little one put on since birth? My little boy has only put on 2.6lbs and he's 9weeks old today. He usually drinks 3-5oz every 4 hours but I feed him on demand so if he wants before I'll give him a bottle. My health visitor basically told me I'm not feeding him enough and I should give him a bottle every few hours even if he's not hungry( which I do if He didn't drink a lot but he refuses to take any and just plays with the teat)I feel like I'm trying my hardest but I feel like I'm a bad mum and I'm doing something wrong.