you don't say much about your grandson - I guess we're kind of assuming a child (above 14 or whatever to actually shoot, I think), but could be 14-24. Are you teaching him to shoot, or does he shoot already, and you're bringing him into the stalking side of things?
I ask because it makes a difference to what I'd suggest. I've been a CCF officer, and with children and guns you need something on them that you can see easily - so I would say over-ear muffs amplivox type ear defence is the minimum. Electronic noise cancelling if you can afford it.
On the other hand, if he is a bit older, and you aren't teaching shooting from the ground up - or you just feel that you can trust he will use them properly, in-ear plugs can be very effective. So long as you get ones with a proper CE marking showing that they can be formally issued as industrial PPE, you can be fairly confident that they will offer good protection if used as instructed.
I use Moldex "Sparkplugs" from
Lucy Dell as for £25 you can get 200 pairs, enough to last for years, and I use for motorcycling and shooting, always have a handfull of packs in my shooting bag, first aid kit and motorbike luggage, and really no drama if you lose or destroy them.
If I did any serious range time though, I would buy some over-ear defence, and insist that anyone I was responsible for used the same.
HTH,
Guy