Weight issues

Hey there everyone! I know I dont post a lot but I would love to be more active here! I was wondering any of you out there are dealing with weight struggles with your littles. My son just turned 4 months today and we took him for his check up. They again are concerned about his weight gain and we have to go back again next month to check it. He was a big baby when he was born and his first visit he was in the 57th percentile. Since then he has been slowly dropping and today the weighed him at the 15th percentile at 13 lbs 11 oz. He recently has been eating 28-31 oz a day and the doctor said he should be doing 30-35 but no less than 30. I've tried everything to get him to eat and I dont want to feel like I am force feeding him. I just feel at a loss on what to do now. We are going to start cereal and purees tonight so hopefully that will help. I just would love to talk somebody experiencing some of the same things!