Never bought boots online, I tend to buy them when they are on sale around local stores. Make sure he tries them on before buying. I play a center back (sweeper), and yeah make sure they are tough boots, I have torn many up through the years in some rough tackles.
Had mixed results with Nike, Adidas used to be good but at one stage they went all nasty plasticy, Im a convert to Pumas now, used to have kings and they were the most comfortable boots I have ever had.
If he has narrow feet, the mercurial line are really good, I had a pair and they were like gloves, just fit perfectly. But they were a bit more delicate though, I went through 2 pairs and they lasted a season each (which is about average for me anyway), both times toe opened up.
I dunno what your fields are like over there, but if they are very soft, you might wanna go the Soft ground (screws). I haven't had a pair of screws for years, our grounds are very firm around here, but this year its rained like crazy and most fields are cow paddocks and the molds just get clogged up.