My 6month old baby feeding problem?

I am a working mother. I pump the milk and grandma feed the baby by bottle. However, the baby doesn't like bottle and cries a lot when feeding. I pumped enough milk, but he just eats very little and gains weight very verly slow recently. It looks like he enjoys everything but eat. Since I have enough... show more I am a working mother. I pump the milk and grandma feed the baby by bottle. However, the baby doesn't like bottle and cries a lot when feeding. I pumped enough milk, but he just eats very little and gains weight very verly slow recently. It looks like he enjoys everything but eat. Since I have enough breastmilk, should I add more solid food to his diet? Right now he has cereal one spoon per day. I always worry he doesn't get enough nutrition. How do I do to encourage him to eat more, especially breastmilk?

More information: he was almost 10 lbs when he was born and keeps 75% of the chart since then. But he drops to a little below 50% just within one month (6 month check at Dr's office). I keep checking his weight and found he didn't gain any weight in past 2 weeks. Is it normal?
Update: Actually, grandma is some reluctant to add more solid food and tries to feed the baby more milk because she believes breastmilk is the best for the baby as long as I have the milk. I am not very sure if she is doing the right thing. The problem is even though grandma works very hard, the baby just take 7 to 10 oz... show more Actually, grandma is some reluctant to add more solid food and tries to feed the baby more milk because she believes breastmilk is the best for the baby as long as I have the milk. I am not very sure if she is doing the right thing. The problem is even though grandma works very hard, the baby just take 7 to 10 oz during a daytime. I think it is too little for a 6 month old baby. At night, I nurse the baby by myself. He normally ask for feeding 3 times at night, but each time is short, around 5 minutes. I feel so upset and frustrated when the baby doesn't eat much. I think I will ask grandma to try the baby's cup or different type bottles.
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