it is up to you. show him to do it properly though. the kids are most likely jealous of him because they don’t have facial hair yet. he shouldn’t worry about it.

I can’t imagine there would be any reason not to let him start now, especially if he is feeling self conscious about it.

Or you could tell him that hipster beards are in now

It depends how much facial hair he is growing. You could start now. I know a lad who had to start shaving younger than that. If he has a birthday soon or it is getting close to Xmas, maybe then is the time for him to be taught. I would speak to his Grandmother about it and tell her to stop teasing him, that she is upsetting him and being cruel. If you want to be politically correct if she gets nasty about it, you could tell her it is emotional abuse.
Back in the late 1980s the teachers in some schools taught their reception class about all types of abuse. (e.g. if you yelled at your child even if you didn’t use inappropriate words it is verbal abuse.—–I kid you not. —–not impressed when a 5 year old comes home from school and tells you that, a few other things and that kids have rights). It was a SA Public School. (They also taught sex education to 5 year olds within 6 months of them starting school–grrr!! Objections by majority of parents of pupils in one class).