Doctor is concerned with babies weight gain..

My LO has only gained 5 oz in 2 months. He is 6 months old and weighs 15 pounds and is breastfed but has been getting baby food. I've been feeding baby food every once in a while 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.