Bad eater

My baby is a bad eater. He doesn't drink much milk anymore. He's on the 10th percentile for his weight and his pediatrician wants him to gain weight. I am currently giving him enfamil AR due to his reflux ( he couldnt keep any liquids in) other than that formula since its thick. Anyway, what advice do you ladies give me? I made him gain 2 lbs but now he eats less than before 😫 in a 24 hour span he drinks 20-30 oz of milk not including solids. He's 7months old. Help with advice. Please. Thank you.