This question is more appropriate to ask of your pediatrician. They have a lot more experience with infant penises than any of us here.

With that said, I have a three year old whose foreskin I don't retract and he's never had any sort of problem. There is a chance of some irritation of the mucosa from urine (same thing happens with little girls' mucosa) but changing diapers regularly, regular bathing, and general hygiene help prevent that. If you notice his foreskin is irritated, let him "air out" without a diaper for a bit (yes he will urinate on your carpet, but this is a good time to introduce him to the potty anyway) and use some zinc oxide or petroleum jelly when you re-diaper him. If this doesn't go away, call your doctor.