I mean it's good advice to just come out with it as swingline and Wolf Pack say but, it can be very difficult for some people to actually do. I've at most hinted to "help me out" or words to that effect, as in shave the sides and back to a number 3 or 4, and trim the top to keep it "tidy, if you know what I mean".
Hairdressers, if they are decent like most people, won't want to start acknowledging the elephant in the room unless you do.
If you want to take that step, as I say it's not a bad idea, maybe try saying it in a different hairdressers? If you feel really embarrassed, go back to the same one, forget the other place exists. But actually I'm pretty sure you'll feel comfortable just discussing it, as mentioned before they are probably aware of the sensitivity of it as I guarantee they've been asked before.
I don't think I'd mention it as I've had some re-growth and I'm sure my regular hairdressers, all of them in there are bound to have noticed, and there's no way I'd want to go down the topic of re-growth. I would guess 90% of people, even if assured that drugs and foam is completely safe for most people, would still just assume I'm wearing a sports bra and have ED, with nuts the size of 2 burnt peas.