Green’s Dictionary of Slang

(US) a lover, a prostitute’s kept man or pimp.

1908		J.M. Sullivan Criminal Sl. 4: Boilermaker—A lover.
1935		(con. c. 1900) AS x 13: candy or candy-kid. One who has a way with women. A dink, a boiler-maker.
1939		D. Maurer Lang. Underworld 117/1: box-coat, a pimp. Also [...] boilermaker.
1942		Berrey & Van der Bark ATS 460.11: boiler maker [...] a ladies’ man, a dandy.