9 month old not gaining

My 9 month old was born full term. No complications. He got sick last month, and has not gained the weight b he co from almost a month ago. He still eats only 3 ounces at a time so still taking a bottle every 3-4 hours. At his 9 month check up we found our 4th ear infection in 4 months and bronchiolitis. His pediatrician seemed concerned about his weight but told us "just make him eat more." We go back in a week to check everything out including his weight. What can we do to help him gain weight??? His eating is still the same so I know he isn't gaining anything.