Shaving the armpits, the pubic hair, and trimming the nails are not obligatory before entering ihram for Hajj and Umrah; not on the man nor on the woman. However, it is among the desirable things to do so that it does not look indecent on them during Ihram, because of the prohibition of trimming and plucking when in Ihram.

This can be found in the book of Al Mughni by Ibn Qudaamah page – (2285) 

It is recommended to clean up by removing the hairs, cutting off the bad odor, plucking the armpits, trimming the mustache, clipping the nails, and shaving the pubic area because they are Sunnah as bathing and spraying of perfume are Sunnah for Jum’ah prayer.

Since Ihram forbids cutting hair and clipping the nails, it is desirable to do these before the ihram.  Lest he may need to do them when in ihram, and will not be able to do them.