The first thing, as a matter of urgency, would be to rule out any medical conditions that may be developing for your son. Fussiness could be a sign that he is developing an infection or other condition that may be causing discomfort or pain.
Have there been any other recent changes? Is your son feeding as normal? Is he pooing/peeing as normal? Has he had any recent fevers? Have there been any illnesses in the family?
It is always worth getting a doctor to check over your baby. Preemies are more vulnerable than term babies and are more susceptible to a range of different conditions, particularly in the first few months. We rush our preemie to A&E at the first sign of anything strange and although we do feel a bit silly when most of the time they tell us she just has a little cold and it's nothing to worry about, I have certainly been grateful for the reassurance and the doctors do understand our anxiety after all our baby has been through.
Medical conditions aside, premature babies do develop differently to term babies.
Term babies spend the third trimester in the womb, a quiet and dark place. Premature babies spend this in an incubator which is loud and bright. They have all sorts of invasive procedure performed on them regularly. This does affect the way their neural pathways develop.
A premature baby doesn't tolerate stimulation as well as a normal baby. They also find it harder to self-regulate their emotions. If there is too much going on for them it can be easy to become irritable and the child will find it harder to gain control over themselves.
It is easy to over-stimulate and harder to calm down afterwards.
If your child is becoming fussy, it would be worth looking at the stimulation he is receiving. Is the environment bright and noisy with lots going on? It may be worth toning things down. Make the environment more simple, less bright colours and less to look at.
What play time does the child get? Is it boisterous with lots of throwing around and activity? Perhaps tone it down, more gentle play and less activity.
This article gives more detailed information : Understanding Neurodevelopmental Outcomes of Prematurity: Education Priorities for NICU Parents
It advises a process of gently increasing a childs stimulation by :
Parents may need additional support to learn to make sure that their
infant is not overstimulated and to encourage quiet, awake, alert
states in which mutual gaze and beginning contingent social
interactions can occur. Parents can begin to gently "play" in
face-to-face communication through singing, imitation of infant
noises, and commenting on infant behaviors so that conversation is
modeled. Repetition and routines of this kind are not boring for a
child.51,52 Gradually increasing the complexity of the speech directed
at the child in multiple contexts, engaging in rhyming games and
repeating and rephrasing infant vocalizations will build language
skills. In all of these interactions, the establishment of mutual
gaze, shared attention, and turn taking are especially important. If a
child does not begin to successfully engage in these behaviors,
intervention services should be sought.53