My son turned 5 last June, and he started Kindergarten this year, and he has done wonderful! He enjoys it, he's excelling, and I have no regrets sending him as a 'young 5'. I would have had serious regrets if I had listened to the pressure around me to hold him back, because there was no reason to hold him back. Being a 5yr old boy isn't a medical disease, and Kindergarten teachers know this. My son has a kid who turned 7 in January in his class, and also two little girls who were 7 in February, and from volunteering in his classroom they are disruptive in the classroom, they are TOO OLD to be in Kindergarten!
I realize your son wouldn't turn 7 in Kindergarten, but if you feel he's ready, then send him! STart a morning routine in early August. If he's never been to a preschool program, then start one at home. Get him to write his name, numbers, letters. Practice with puzzles, coloring books, counting with little objects (my boys LOVE buttons!) and working on shapes and colors! At 4, he should already knwo his alphabet, colors, and numbers to an extent, so just build on that. Don't drill him, but make it fun, and he'll be ready for Kindergarten.
I really do think that a year of preschool is very important for kids to get a good start into Kindergarten, because they have that exposure to the classroom setting. So if he'sn ever been in a preschool, then I might consider doing a yearof preschool before Kindergarten. But if he has has preschool, then go for it, send that boy to Kindergarten!