First, ya need a hydrometer, to measure the specific gravity of the water. Brackish bumblebees do well at a specific gravity of 1.005-1.008 or so. Ya just add marine salt for a saltwater tank until ya hit that range.
Ya may not want to switch him back to FW. It isn't as simple as that. Not all bumblebee gobies are able to survive in FW long term. The three most common species in the trade, Brachygobius xanthozonus, B. nunus and B. doriae all do best in brackish. In fact, B. xanthozonus needs to remain in brackish for long term survival. These three species are nearly identical and the only way to reliably determine species without killin' the fish is by countin' anal fin rays. B. doriae and B. nunus both have one anal fin spine and seven rays, and the first black band rarely encircles the body completely. There's some debate whether they are actually different species, and I'm not sure what the current staus is. B. xanthozonus differs from the other two by the number of anal fin rays (one spine and eight rays) and the first black band behind the pectoral fins is generally broader and nearly encircles the body completely.
Hope that helps a little. Maybe ya can tell which species ya have. If not, yer best keepin' him in brackish water.
WYite