I wouldn't have him do Saxon math next year. Saxon has a reputation for being boring-- going through the same workbook two years in a row can only make that worse.
Honestly, I wouldn't have him do formal math at all. He'll get that when you send him to school. While he's home, I'd focus on stuff the school couldn't do. Instead of buying curriculum, maybe get a museum membership, and visit once a week. Cook with him, let him play lots and lots.
If you feel like you need a math curriculum, I'd look for something very different from Saxon-- maybe RightStart?
My 5 year old is doing MathMammoth Blue Books which teach each skill individually. They start with addition 1, but as pp said he mightbe getting a repeat of that with his Saxon next year. Timberdoodle, which is pretty old homeschool supply company, recommends Mathmatical Reasoning for this age...not a textbook but helps them develop skills.
My son has enjoyed the Horizons Kindergarten workbooks very much. It's colorful and engaging with lots of variety and review. He also likes the Singapore Early Bird workbooks, but Horizons suits us both the best. You can get Horizons at cbd dot com, as well as aop dot com and on ebay. CBD has the best price right now.
If I were you I would skip the Saxon. I say this because your son will go to K, just a year later than he could. If you were going to have him start the ps in 2nd grade (and you knew this) then I would go for the Saxon.
In your situation, I would use the free worksheets form homeschoolmath.com (the math mammoth person) or one of her books. But that would only be secondary to games like shut the box, the RightStart Math Games Kit, measuring everything around the house (or do a measurement scavenger hunt), explore volume with real containers and water. . . how many times can we fill the cup up and dump it in this quart jar, cooking, etc. My 6 yr old has always wanted workbooks, but she does them on her own terms. So, we do all sorts of math throughout the week and then she might do a workbook page once or twice. Our "game" style math actually works her at a higher level. She doesn't mind making mistakes through the game. She hate mistakes with workbook pages though. So, they are mostly review with an emphasis that a mistake is ok--we learn through our mistakes.
For reading, I would go with what works well with you. Many kids benefit from having things presented several different ways. Many will learn to read regardless of what is being done to teach it. I think that you reading to him will still be the most important thing. If he wants to learn reading now, great! Follow his lead, but again, learning to read doesn't need to be workbooks. My 6 year old does really like the explode the code workbooks though. She did the ready, set, go for the code when she was preschool. Now, she independently (for the most part) does explode the code. On her own terms though (again). For actual practice writing, there are lots of writing opportunities in ETC. We don't do all of them. We also like the Draw Write Now books (drawing and writing). But mostly we like to make writing part of our day vs a chore to do.
Mom to three very active girls Anna (14), Kayla (12), Maya (8).