Baby is not growing as much as he should

Is

My baby was born almost three weeks early and was a very small baby. I've exclusively breastfed him and his growth curve has gone straight upwards until he was about six months old and we started introducing solids. He doesn't like food much, he just recently, around two weeks ago, started actually eating food. He used to nurse every two hours and then around six months he started eating about every third hour. Now he's eight months and at his last appointment they said it's normal for babies around 6-8 months for their growth curve to slow down a bit, but that he seems to be under his curve now so we got a new appointment for next week. What will they do if they decide he's not growing well? Will I have to supplement with formula or what? I have lots of milk, I'm still overproducing. Could this be because my milk isn't as nutritious anymore or why isn't he growing as much as he should? I'm so confused and scared.