My premmie DS1 (6w prem, 1.9 kilos) had huge issues with constipation, particularly because he had what the paed described as far too much milk.

I wouldn't be giving him milk any more. Give him water instead. Eventually he'll get onto food.

With regard to the constipation, there's a child friendly laxative called Parachoc - my son was on this for over a year. It softens the stool so it's more easy to pass. Be aware that once the bowel has been blocked, it can take up to a month to "ease" the congestion and the inflammation which has been caused by the constipation. Your DS may also have anal fissures which cause significant pain on passing stools.

However, before you do anything, I'd take him straight to a paed.

Good luck.