I just wanted to gauge whether it was cruel to cut his hair.
Don't be silly. This can't be a serious question.
No, don't shave it all off 😵💫 talk about drawing more attention to him.
Cut it to a reasonable length, slather in conditioner & treat using a nit comb, Nitty Gritty only, as others said.
It will take ages the first night, yes. Just accept it.
Do it 2 or 3 days later (no point in doing it the next day, you are waiting for any eggs to hatch), it will be much shorter. Keep going with this until you've fully cleared it.
No need for treatments. They are messy & less effective.
And without being harsh, be more of a parent. 3 weeks is awful, to have let your DS endure this.