I always thought "jeans' pocket" was correct. Since you say, "the pocket of his jeans."
However, when I searched for "his jeans' pocket" on Google Books, the results returned the version without the apostrophe.
Why is that?
Note: I have the same question with "his shirt's breast pocket" vs "his shirt breast pocket."
the 's form should be used to form the possessive of a singular noun. However "jeans" is treated as plural (just as "pants" is) and so the possessive would be jeans'. But when one indicates "the pocket of a pair of jeans" one usually uses "jeans" adjectivally, so it modifies "pocket" giving "the jeans pocket" and not "the jeans' pocket". Indeed the latter form is so unusual that I can't recall having seen it. Similarly, in "the shirt pocket" shirt is being treated as an addictive, not a noun to be put in the possessive form. However, when referring to a particular shirt, a possessive form might be used:
My blue plaid shirt's pocket.
Whether to use a possessive form, or to use a noun as an adjective, is just a question of usage and custom -- either is grammatically correct, but in a given case one may be natural and the other not. It might also vary in different varieties of English for some words.
The Texas criminal court in a matter regarding a pickpocket (did he take it from the victim's pants' pocket or pants pocket?) in 1908 decided to go with and without the apostrophe in the same brief. https://www.google.com/books/edition/The_Texas_criminal_reports/B3QyAQAAMAAJ?hl=en&gbpv=1&dq=out+of+his+pants%E2%80%99+pocket&pg=PA543&printsec=frontcover
