T.S.
I'm sure you can start homeschool at any time. Even if he's "officially" in preschool there's nothing to stop you from doing Kindergarten academics. Unless he's an age for mandatory school, you don't even need to register as a homeschooler because no one expects him to go to school at all.
That being said... don't underestimate the need to be socially ready for the grade if you plan to send him to regular school at some point. My son is in kindergarten this year (turned 6 in January) but we TRIED to start him at a private school last year because he was already academically at an end of 1st grade level. It was a mess and I pulled him out after two weeks and sent him back to a play based nursery school to have another year to develop those social skills.
Now... he's doing great in kindergarten. He's even more advanced academically, but his teacher differentiates for him and he is rarely bored. Socially, he's still got stuff to work on, but he's more middle of the pack and is able to make friends.
If your only concern with waiting for kinder is that he will be too advanced, I'd recommend holding off (I know that wasn't your question). Do the kindergarten books at home, but let him stick with his peers at school. If he's truly too advanced, you can always do a grade skip later, but in the mean time wouldn't you rather he feel like the SMARTEST kid in class than the immature one nobody wants to play with?
HTH. Good luck with whatever you choose.