Does he help you with the twins? Does he do chores? Give him more responsibilities to help him feel good about himself. Maybe enroll him in a karate class, or baseball (or some sport).
I think if he has gone to preschool then he should be O.K. in all- day Kindergarten. If you put him in the all-day kindergarten, there may be more issues when you change back to the other school -- no familiar faces, etc.
Give him a good breakfast like oatmeal, and maybe consider giving him vitamins that give him energy, like vitamin B. Maybe reevaluate his bed time. Make sure he is getting 12 hours of sleep. I believe they give them nap time in all-day Kindergarten? Call and ask your school. The office should be opened by now.
Tell your twins that they have such a big brother that gets to go to school--he'll hear you talking about him positively. It works!
From now on, talk only positive to your son, tell him what a big boy he is about everything, make the teachers aware of his situation, do the same routine every morning, and make the goodbye very short (Don't allow his crying to get to you and keep your emotions under control by reacting with positive words. Be matter-of-fact, tell him that you know he can do it, give him a kiss goodbye, turn around and leave).
I was glad my kids had late birthdays because they were more socially ready for school being almost 6 when they started. I really think he'll be just fine after a few weeks. If you and the teacher really feel it's not working then you can take a different plan of action at that time. Maybe even hold him back another year?