I have a lot of experience with this and I got a lot of intervention for my son because he was never on the charts with weight and even though he was meeting his milestones, his height started dropping so we took a lot of measures. I am not sure what's right for you but here are your choices.
You can see an occupational therapist and have sessions with the OT to help him overcome any eating issues he might have. This has been very useful for us because he hated anything with a moist texture and only ate dry things and she was able to get him to eat mac and cheese which is a big thing for him.
If you think his stomach is not processing well and he can only hold so much food, they can give him Reglan to help his stomach work more efficiently. This did not help my son and actually gave him reflux but for some kids, it has been successful.
See a dietician. Some things we've tried are: put heavy cream in his milk, give him Pediasure, give him Kid Boost 1.5 (this has the most calories almost triple that of milk), add a flavorless powder called Duocal in his food which adds about 50 calories a teaspoon full, add coconut cream to his food, etc. Also, if he is not eating much, it's okay to give him milk or Pediasure even at night so he can get all the nutrition he needs. When they are this young, they need the fat and nutrients for their brains and bodies to grow.
Extreme options: feeding tube, get an endoscope, get a GI tract x-ray.
My son was premature and always gained weight slowly but once he started losing weight, we had to put him on a feeding tube for two weeks. It's horrible and I question if it needed it to be done or not but he did gain 2 lbs a month and now he is on the charts, between 2%-5% most of the time but at least on the charts. Hopefully, you won't have to do this because it's heartbreaking. Good luck.