I think our son is doing RX drugs (zoloft, ritalin, xanax, etc.) How long do these stay in the bloodstream?

You are looking a pretty short half-lives for the aforementioned medications, especially the Ritalin. Seriously re-think throwing away your marriage. You will need the support if your son is using. Accepting that people are at different stages of acceptance is critical. Good luck to you and your family.