If a male converts to Islam is circumcision necessary or not?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Circumcision is legal for a new Muslim unless there is fear of some harm.
There is agreement among Muslim Imams that circumcision is a legal confirmed action for all Muslims. Circumcision is a duty according to Imam al-Shafi'e and the well-known opinion of Imam Ahmad .
The Prophet Ibrahim (Alaihi As-Salaam) had circumcision at the age of eighty years.
As for a new Muslim, evaluating harm of circumcision should be based on the opinion of reliable doctors of medicine. In other words, circumcision may be delayed to winter if it might cause harm in summer or it might be delayed to sometime that suits the new Muslim in order to avoid some harm to him.
Allah knows best.