C.T.
Most schools don't have prek unless your low income. And like R. said above- if he's not 5 by the cut off date he won't start school this year. But also YOU can decide to hold him back another yr if you want- even if the cutoff day is after his birthday.
My oldest starts kindergarden this year too, so I'm with you on the "overwhelmed" feeling. We live in Birdville ISD and they had a Parent roundup day in April. Most schools have early registration. And since you prob. missed that you will need to check the ISD website for regular registration.
Good Luck