My preemie daughter had reflux bad. Try and keep him upright for 30 minutes after feeding. Hold him over your shoulder, put him in the car seat, let your husband hold him, etc. It helps settle the milk. You can have him sleep on an incline if possible (couple of blankets under the mattress by his head). They eventually grow out of it. if you think his belly is cramping, try kangaroo care which is him naked (or in diaper) laying on your bare belly or chest. The warmth of your body helps sooth his.
Good luck Rachel.