The ones that they sell in Games Workshop shops aren't bad at all. They also have brushes for the different techniques they recommend, like dry brushing and washes. If you have a GW shop near you, it might be worth chatting with the staff in there, they are very helpful.
It depends how serious he is getting. If he is getting good, then good quality brushes like Rosemary & Co are worth investigating (buy online). Personally I wouldn't use anything other than Winsor & Newton Series 7 but they aren't cheap. However, if looked after they last a long time.