Sensitivity can decrease in a circumcised penis, and it's a common reason for older men to restore their foreskins. The thinking is that dead tissue builds up on the glans penis. With a foreskin, there would be moisture present to exfoliate that tissue, but without a foreskin it just dries and remains there.
Foreskin restoration (google it) is most commonly done without medical procedures. All you need is your hands to pull the skin forward, though it's possible to use tape or weights to hold it in place. It takes a long time, but it's worth it if you want a foreskin.
I actually don't think that's true - I was never circumcised, and the sensitivity of my penis has decreased over the years as well - I think it's a function of getting older :-) there are complicated expensive procedures to reproduce male forskin, but I'm not sure if that works or how they do it