I would think that is pretty normal? DS isn't yet 2.5 (but he's getting pretty close to it) and he knows his colours, his numbers (but skips a few - or goes backwards instead) and speaks in sentences and is about 80% understandable. BUT - his speech is really good so we have been told (must be cos his grandparents WON'T STOP TALKING!!!)

My sister had speech therapy, but she was closer to 4 when that happened, and that was because she didn't pronounce ANYTHING correctly - not just drop the s's.

I'm pretty sure he sounds normal though - as always -get him checked out if worried though.