Sounds like he has a "tight" foreskin. Good for him!
http://www.cirp.org/library/normal/aap/
Here is what Dr. Fleiss wrote about it in "What your Doctor may not tell you about circumcision"
Page 194 (It's a great book you should have it handy)
The fact of the matter is that "tightness" of the foreskin is a safety mechanism that protects the
glans and urethra from direct exposure to contaminants and germs. The "tighter" foreskin also
keeps the boy's glans warm, clean, and moist. As long as your son can urinate, he is perfectly
normal. There is no age by which a child's foreskin must be retractable.
Be on your guard, Never let a doctor or anyone try to retract your child's foreskin. Optimal
hygiene of the penis demands that the foreskin of infants and children be left alone. Premature
retraction may tear the skin of the penis open, causing your child extreme pain. There is no
legitimate medical justification for retraction. The child's discomfort is proof of that. (END)