At war with myself

I feel like I'm at war with myself. My son is 9 weeks old and was born on his due date Jan 30 at 8 lbs 15 oz. We exclusively breastfeed now, but he has had a bottle a few times when i was away and i was getting 3 oz pumped. He was 8 lb 8 oz when we left the hospital. At his 3 day check up he weighed 8 lb 3 oz and a week later 8 lb 2. We had to go in for another weigh in and he weighed 8 lb 4.5 oz so the nurse said he should be gaining now and that we didn't need to come back. Fast forw to his 2 month check up and he weighed 9 lb 8.5 oz... the dr doesn't know why. Hes a very happy baby. He eats 30 min at a time every 1-2 hours or on his demand, he doesn't sleep long stretches during the day but at night he sleeps 3 and 4 hours. She told me to take fenugreek and follow feedings with a bottle of formula. Most feedings he won't eat more bit if he does its an ounce we force in to him. He pees 8-10 times and 4-5 poop diapers a day. He doesn't look like he has poor weight gain...so I'm not sure what could be wrong.