Can a man shower or towel himself after using the mikvah on erev shabbos or erev yom tov?
There’s an inyan for men to go to the mikvah on erev shabbos.
It is fine to shower and towel after the mikva for men.
Although there is a rabbinic enactment against showering after the mikva, due to the concern that people will say the shower helps and not the immersion, this does not apply to mikva for men, which is not (nowadays) an obligation. This is ruled by a number of authorities (Pe’as Sadcha 105; Shut Maharashda 70-71; Zeh Ha-Shulchan 88; Az Nidberu Vol. 6, no. 72, sec. 10 – among others).
Another reason for leniency is that even a shower provides a degree of purity, in the option of 9 kabin.