I changed this from otoko_no_ko to genderswap, because lower body aside, "he" appears to have very significant breasts. Please revert if I'm seeing this totally wrong.
I'm no Twitter expert, but the only maybe-relevant artist commentary I can find on the post there is the one that refers to the pictured character as a "son". That's not enough to convince me that this should be tagged 1boy, etc. despite appearances, but meh.
It´s tagged with "crossdressing" on his patreon post And, he´s arching his back, givin the illussion of big breasts, but whose are just his manly muscles. Way to much maybe, but in this case, he´s really 1boy.
You could just say push-up bra. Seriously, those things are powerful. Any young man equipped with the right bra could pass looking a B or even C cup with the right amount of posture and physique.