I'm afraid I can't write a story for you. Only you can do that. What I would say is that it's important to have a set of clear ideas about your character and checklist such as this might help:
1) Cock size/dimensions - what are his dimensions?
2) Clothing. What sort of briefs, pants, boxers or pyjamas is your character wearing? Does he like them? Even if their fairly tight fitting I think they should be comfortable and your character should be able to reach his cock easily.
3) Masturbation. How often does your character masturbate? Does he cum every time or sometimes stopped short of cumming.? Where does he like to wank/cum? Does he do it so often that he sometimes runs dry? Has he ever been caught at it?
4) Sexuality. Is he straight, gay, bi, ambiguous or uncertain of his sexuality?
5) Sexual Experience. Is he sexually experienced or is it a first time? Is his experience only of straight 'vanilla' sex or does it include oral and/or anal?
6) Surgery. Has the character ever had any operations on his penis - either for cosmetic reasons or medical ones. Hypospadias is a condition affecting about 1 in 100 guys (of all cock sizes) where the opening is sometimes slightly off centre or on the undershaft. Sometimes it's corrected with operations but mild cases are often left uncorrected. Matt Majors, an American porn star, has it.
7) Non-sexual cock use. Not eveyone's keen on pee but it's one of life's necessities and a good quick way to get a guys penis out of his pants as well as being a natural function which some people find erotic. Also it adds an element of realism. I find big cock stories wherre the guy needs to do a wee at some stage far more believable than wall to wall sexual activity.