I think you will actually be OK starting in R&S 5. It will help though if he has some grammar background. The R&S web-site suggests starting with 5 for anyone past grade 5 and starting with R&S grammar.
My ds11 has done books 2, 3, 4, & is now almost 1/2 way through level 5. Level 5, for him, has honestly been a breeze because he remembers levels 2, 3, & 4 and the first 1/2 of level 5 has been a review of those first 3 books. In fact, the diagramming 1/2 way through level 5 is easier than what he was doing towards the end of level 4. It has been good to review and see how much he has retained, but I can see why R&S says that older students can start with level 5. It is obviously moving through the material more quickly than the lower levels, but I think he will be fine. The first few lessons on diagramming in level 5 really went back to the basics almost as if they had never had it before. Whatever, grammar he has had though will be helpful for him.
Rod and Staff is so beautifully clear in their explanations of grammar and thorough that I think you will like it. I know we do!
P.S. I just noticed in your signature your son is age 10. At age 10, without having used R&S previously, like the others I would recommend R&S 4.