My daughter was afraid to start too. She was a little shy and had been at daycare/preschool for three years and loved all of her teachers. Kindergarten is a big jump for a little person because it's the start of regular school when you're supposed to learn and be grown up. It doesn't have the freedom and youthfullness of preschool.
One thing that helped my daughter and that your son may benefit from is to acknowledge his fear. "I understand you're scared, and that's alright. We all feel scared sometimes and you're starting a big school year this year. I'll be here after each school day and we'll tackle it together." I think it would be like a hug for him if he knows it's normal to be nervous about new experiences, but you'll be there for him. It really helped my daughter and it didn't take her long to start enjoying kindergarten instead of being afraid to go. And please let us know how he does. :D