There is only one way to find out which are the best brushes, and that is by using them. You will get a different answer from most people answering your question, because they all have their own favourite brushes....
You can not go wrong with Windsor and Newton, and they do a large range, not just the very expensive Series 7. Try Artist Watercolour, or Proffesional Watercolour, they are great brushes at a reasonable price. They are in my jar next to the bench, and are the heavy lifting workhorses. Most are years old, and still doing their job well.
For finer work, detail and delicacy, I love my Broken Toad Kolinsky Sable brushes. Hard to get now (If they are still in business), but a superb shape and spring. Hawk Miniatures do a good range, and at a very good price. I have been using their brushes more and more, as my Broken Toad brushes are getting older.. The short shaft is great for mini figure painting, your Grandson might like them for painting Warhammer.
Windsor and Newton series 7. I have a few. I use them sometimes. I think they are good enough brushes, but over priced, and over rated. I prefer the cheaper Windor and Newton brushes.
I prefer (Kolinsky) Sabel, because they have more feel, and seem to hold more paint, and release it better, than Synthetic brushes. Having said that, Synthetic brushes do a good job, but in a slightly different way. I have a set of Broken Toad Synthetic brushes too, and I use them, but not as much as my Kolinsky ones.
Long post short, buy a brush or two of each brand, and find out which one your Grandson likes best.
Oh, I meant to add, I only use Games Workshop brushes for really destructive work, like washes and mixing... The ones I have came with paint or figure sets, I would not spend my money buying one. Truly mediocre quality, very poor shape, and very little control. They certainly don't last. Very poor value for money.