Can a father and son go to the mikvah together
The Rema says that a father and son should not bath together, which would include going to the mikva together. The reason is because it will cause him improper thoughts. (This halacha also includes not bathing together with one’s father in law, step father, an brother in law). If they are wearing underwear it is permitted. There are poskim that look for reason to explain why many people are not careful about this, but they say that essentially one should be careful about this. There are however a few lenient factors brought in the poskim. A father may go to the mikva with a young child, (there are different opinions in the poskim as to the age, some say 9-10, others say until bar mitzvah) since he won’t have an issue of unwanted thoughts. Others say that if they are in different parts of the mikva and they might only meet for a very short time, then it is permitted.
Pesachim 51a, Rema Shulchan Aruch E:H 23-6, Aruch Hashulchan E:H 23-8, Likras Shabbos 1-chap. 7 ftnt 66, Lehoros Nossan 1-61, MinchasYitzchok 4-62, Om Ani Choma (Chidosom Mishta’ashosh 187). Various Halachos that are applicable to Shabbos pg. 23, Chut Shani (Shabbos) 3 pg. 284, Derech Sicha 2 pg. 475, Ashrei Ish O:CH 2 1-22 ftnt 31.