It is important to mention to you, but at this point, it is not a delay, unless he has other issues that cause you concern too and the fine motor accompanies speech, language, behavioral, visual, cognative, social or sensory concerns.
I would take this as information right now and keep an eye on his fine motor skills and act sooner, rather than later if they do not improve by the time he enters Kindergarten and has formal instruction that is appropriate for his age and development. Preschool is not necessary nor is formal instruction at this age, so if he makes the milestones once he gets to school, he is fine.
If you see other things, or he seems angry or frustrated by his inablity to produce writing when he trys hard, then you might consider an evaluation with an Occupational Therapist.
The good news is, if he is just not interested at this point, all the early gains that his peers seem to be making will disapear very quickly as those children without accelerated instruction will catch up rappidly, once they are exposed to the instruction at a developmentally appropriate time.