I am the opposite. My daughter is currently 4 with a birthday in Feb and I would love nothing more than for her to go this year with her friends (I have cared for 4 other kids that are all going to be in the same class this fall - one is her best friend). She is no further behind them (actually a little bit ahead of 2 IMO) but unfortunately I have to wait because she is a month and a half past the cut off!

You know your child best, if you think it would be beneficial to send him this year then go for it. If he will struggle then wait a year and maybe find a pre-school program that he can attend.