I don't know the name in English since I read it translated in Vietnamese. But I can tell you some details.
Basically, MC died by random accident and reincarnated in a novel he just read before he died. When he opens his eyes, he can see but can't see his arms or legs or move at all. Until the ML picks him up and put him on him (cz he's his underwear now).
MC tries various ways to see if he can escape his fate as an underwear or tries to see if he can move at all. He can, but he can only fly lmao (so imagine a flying underwear escaping).
He later turns back to his human form but that's near the end, he's still able to switch back n forth between human form n underwear form.
Sooo, if he decided to hide in his underwear form, the ML just hold him n put him around his d*** and start jerking off
I like this one. Seme becomes uke's dick
https://www.mangago.zone/read-manga/chinko_no_tsubuyaki/
This is the link I've found:
https://booluyyly.wordpress.com/2015/12/03/xuyen-thanh-ao-lot-cua-nam-chinh-thi-biet-man-sao-thuong-liet/
Guys, this is nothing. Y'all need to read more Chinese mlm novels. I once read a novel where MC (bottom) becomes the ML's underwear. YES, UNDERWEAR and they still bang each other