Most listings I see are for spas. I find the men in Singapore to be generally gorgeous, so I wouldn't limit myself to expats. They're very body-conscious and fitness focused. I think the spa I visited on a past visit is no longer in business so I'm not too much help.
Utopia is a good resource for Asia
https://www.utopia-asia.com/singmasa.htm
TravelGayAsia has some listing as well
https://www.travelgay.com/singapore-gay-massage-spas/
