How old is your grandson, Chippy? That could influence the choice, too.
Also, since he paints Games Workshop, it might be worth it to look at their products. Though they might be more expensive than comparable brushes from other brands.
Personally, for my water-based acrylics, such as Andrea, Vallejo Model Color, Lifecolor, and craft-store paints like Folk Art, Americana, and Apple Barrel, I prefer natural fiber brushes, because I find that with care, they last the longest. The big thing is the shape of the brush. I like rounds, and I have 0/0, 5/0, and 10/0. I think of them like old fountain pens; the body is the reservoir and holds the paint, and the fine tip is like the nub. I like the precision. I like using a wet palette, too, but that might be more than your grandson needs. That's why I asked about his age, too, to keep things appropriate to his age.
Hopefully this is useful info, too.
Prost!
Brad