Prepuce or foreskin is known to cover the glans of the penis completely in childhood , but as genitals develop and erections occur this foreskin gets pushed aside by the erect penis and hence the aperture goes on increasing to enable on to retract it completely, for urination, and sex. If you are able to see small part of glans at erection, you can use masturbation to make the skin go back and to increase the aperture. Use lots of lubricant in the area between the foreskin and the glans, then slide the foreskin backwards, this will help the foreskin to go slowly over the glans top. This done repeatedly can help the foreskin to become pliable and it may then go behind. If this doesnt happen kindly use some stretch on the foreskin aperture to enlarge it. If this too fails, you can go to a surgeon and get circumcision done to increase the aperture. Circumcision does not mean cutting the whole foreskin. You can tell the surgeon your demand to keep it but cut only that much which causes foreskin to slide back with ease