I see you are in Puyallup, so this might not directly apply, but:
In Oregon, once they start school, if they need individual intervention it's called an IEP (individual education plan, I think that is), and yes, that is separate from an early-intervention plan ... but still the same concept.
Last time I checked (several years ago), parents (in Oregon) couldn't be prosecuted for keeping their children out of school until the child was seven. There is a lot of research to support NOT starting a kid into kindergarten until late-five-years-old or even six. The basic rule used by Waldorf-method schools, which evaluate grades-readiness by maturity rather than age-at-enrollment, is that most kids should not enter first grade until late-6-years old ... so, birthdays June 1 and later undergo a heavy scrutiny to assess whether they should 'stay back' from what our broader society says is the 'right' grade for them. Word is, some Waldorf schools are moving that back to May 1, now (I suspect because our society is so generally not geared to successful childrearing that kids are, on average, less developmentally in gear now than they were 100 years ago).
The piece of the research that I read (and a lot of it was from standard kindergarten teachers/researchers, not Waldorf ones) that resonated most acutely was that even if the summer-birthday 5-year-old-kindergarten-start kids did 'fine' or 'well' academically, their rate of suicide and suicidal thoughts in *high school*! was significantly higher than their non-summer-birthday classmates, whether or not those classmates were 'successful' in school. (I was screamingly 'successful,' in all the ways society measured, but I (I now see) had missed several necessary steps in my development as a kid, and I knew I was broken and noone could help me figure out what was broken, to fix it ... I've done most of it now, but at the expense of failing at my marriage :(. )
BOYS ESPECIALLY may just need another year, and that is OK, and it is supported by alternative and mainline research (not, unfortunately, research that is paid much attention to, in these days when so many people expect school to be free day care "as soon as my kid turns 5!"). Educational research and neurological development research.
NOW, if you can work the system to your son's advantage (get free therapeutic help that doesn't force him to grow but helps him grow), I for sure think enrolling him--if he *can* be successful--is a great plan. The key thing, in my thinking, is to keep him in environments and with people who will help him see that he can achieve things. If a classroom of 'normal' kids will discourage him from liking school, that isn't a pattern one necessarily wants to set in his brain. If the classroom of 'normal' kids would help him see what he *could* be doing (my sons particularly sometimes haven't realized they *could* be trying something, until they see others their age doing it), well, then, pop him in there.
Personally, I'd check the truancy laws for Washington (so you know if you can just pull him if it isn't working or if you have to have an approved homeschooling plan first), and then enter into fact-gathering and registration for whatever therapeutic offerings you have available (or applying for transfer if you think that would be best for your son). In Oregon, homeschoolers still have a right to participate in the academic and other resources (therapy, sports, clubs) provided by their 'home' school or district ... so you might be able to work the system for half-days instead of whole-days, or two days plus a therapy day, or ...
And I expect any specialized plan will take a lot of fight. But your son only starts school once, and an initial bad experience on something society expects him to partake of for the next 13 years isn't a good plan (consider your long-term family emotional life around this issue, too).
Plus, I am a big fan of forcing the schools to deal with the reality of children and not just the 'ideal' education dogmas of the moment.
Your child is what matters, and don't ever let ANYONE tell you differently, because they are either lying to you or themself (or their boss).