Doctor is concerned with babies weight gain..

My LO has only gained 5 oz since his 4 month check up. He is 6 months old and weighs 15 pounds and is breastfed but has been getting baby food. I've been feeding baby food and then breastmilk after but learned today that I should be feeding breastmilk first then baby food. But doctor seemed really concerned about his weight gain and said he may not be getting what he needs with from the breastmilk and said to offer formula after the breastmilk. But he's never taken a bottle (I had never even offered him a bottle or sippy at all) and I really don't think he will take the formula. I don't really want to give him formula either but will if I have to.. Doctor wants to see him back in one month. But now I'm also worried.

He is concerned because he isn't gaining weight like he should be..

Thank y'all.

The issue isn't really his actual weight it's the how he's gaining it. He's not gaining much at a time and just doubled his birth weight. He's almost 7 months now and the fact he's only gained 5 oz since his 4 month check up is what is worrisome.