Nick Young Serenades Baby Mama on Fancy Date Night

You know how we know things are getting serious between Nick Young and his baby mama (besides the 2 kids they have together?) ... he's singing to her!!

Swaggy P took Keonna Green out to Catch in West Hollywood -- the hottest restaurant in town.

But after dinner, Young busted out the pipes and sang a little B2K and old-school Beyonce.

Remember, Swaggy got back with Keonna when he knocked her up last year -- while still engaged to Iggy Azalea.

Iggy's moved on ... and Swaggy seems happy -- so, for the best???