Late Term Preemie- Grunts/Strains/Cries

Amanda • First time mom. Married to an amazing man. 3 meowing furbabies. Roadie. Entrepreneur. Lexus IS350 F Sport enthusiast.
Hi ladies! 
I have a late term preemie. My son Nicolas was born at 35 weeks on March 18th. 
He is calm and quiet all day, but as soon as night comes he grunts/strains/cries alllllll night long. Even with gas drops. 
I have tried laying him (supervised) on his left side, using a lower flow nipple to bottle feed him breast milk, pumping his little legs and massaging his stomach in addition to the gas drops- nothing is helping him and it kills me to hear him like that AND we aren't getting any sleep. 
Is this normal? When does it stop? Is anyone else going through this?