It stops tickling after you get used to it. Which takes a couple minutes. Also, hair grows slowly, so every time he has a spurt, he may get used to it.
Also, after a while when hair grows (at least for me) it isn't as noticeable if you've gained a half inch. Sure, if you went and measured it you'd know. But sometimes it is gradual and sometimes it seems to just shoot out the roots and grow like *snap*