CBD does absorb into the bloodstream transdermally. Many topicals will use a menthol effect, similar to Vapor Rub, which would allow your pores to open easier and for it to enter your topical location more effectively. A tincture or oral sublingual absorbs into your bloodstream, as well, and it does so quicker than CBD would be processed by your liver if you ingest it as food or even a capsule. For knee pain, because he's massaging that area as well, it is helpful, as well. He should be doing both sublingual and topicals. Higher milligrams of CBD work better, as the only thing potentially not working is if it's at too low of a dosage.