Is there a male equivalent of "dowager" with regard to British titles?

Solution 1:

It is still "dowager." In instances where a king consort has been widowed by a queen regent, he is referred to as the dowager king or dowager king-consort, as exemplified by Ferdinand II of Aragon when his wife Isabella I of Castile preceded him in death. From 1506 to his death in 1516, in the kingdom of Castile, Ferdinand II was referred to as the Dowager King-Consort of Castile or King Dowager of Castile, ruling Castile as regent, because his daughter with Isabella I, Joanna, was not yet of age.

Here are a few of other examples: Dowager Duke Eastman, Dowager King Consort, King Dowager Beets, and Dowager Duke.

Solution 2:

To better distinguish Queen Elizabeth, a Queen Regnant, from her mother Queen Elizabeth, Queen Consort to the late King George VI, the latter was styled Queen Elizabeth the Queen Mother. She was a dowager queen, but was never referred to or addressed as Queen Dowager.

If the present queen should die before her consort, HRH Prince Philip, Duke of Edinburgh, he would continue to be (and be addressed as) Duke of Edinburgh, not Prince Dowager.