My brother wants wetsuit boots for his birthday and I dont know what thickness/make to buy him. I know he kitesurfs most of the year round off the south coast of England in a 5mm wetsuit and is fed up of frost bitten feet. He wants split toed boots but that's all I know. HELP!

you will find most boots are 3mm
you could find diving boots which are 5mm but they may be too thick depending on how much his straps adjust on his board.

split toe mauwi magic if thats how its spelt; i have some, although i bought a pair of Alder 3mm wet suit socks and wear those in preference as they give me more feel in the foot straps; they do however only last for one winter before they get holes in them. i am on my 2nd pair and love them.
boots from powerkiteshop.com
alder socks from cornwall, do a search in google think they coast me about £14 last week.

Most boots that I've ever had have been 5mm.. And most that you see in the shops for winter use are 5mm as well... I'm usually more bothered about the thickness and flexibility of the soles than the thickness of the neoprene though as I like nice thin, flexible soles so that I can still feel the board beneath my feet.

If you want, you can support Forces-of-Nature when you buy them. Just go to the store here and types 'Boots' into the search field and you should find a nice selection of wetsuit boots. You pay normal Amazon prices and get them from Amazon but Forces-of-Nature gets a tiny bit of commission too, all of which helps keep the site up and running... Same for anything else you want to buy from Amazon as well.

Cheers.
Al.