Easiest way to go is to get a small digital multi-track recorder.

Track 1 = synth left
Track 2 = synth right
Both of these are sent to the PA system.

Track 3 = click/drumtrack
This last one is sent to the drummer through headphones or in-ears... if the drummer prefers tracks 1 and 2 can be added also.

A lot of bands do this when they use pre-recorded backing tracks while playing live.

This does require your drummer to be able to play along to a click track. If he's not capable of doing that, I see no other solution than getting a keyboard player.