What size would my son take in this boot if he wears a size 10 or 10.5? Does .5 mean half size?
Thanks

a size 10 U.S. is roughly a size 28 - 28.5 in mondo sizing. I have worked in the ski industry for 6 years and I have sold many pairs of Kaos boots. Also a tip for you, if the a ski boot is a tad too big throw some Dr schols footbeds in the liner or if the are too small take the footbed out.(not recommended for a pair of boots that you will buy). Since your buying these boots online I would also suggest to go to a local shop and size the boots to the feet

with the whole size and the half size it is the same shell and liner. but i truly would get your son sized by a professional -- you don't want ski boots fitting like a shoe.

I would whole-heatedly suggest you take your son to a reputable nearby ski shop (if there is one) and have him fitted in boots. Ski boots can be pretty finicky when it comes to sizing. I'd recommend checking the listings here: http://skibootfitters.com/FIND_SHOP.htm, find a shop near you, and take your son. I know that doesn't really answer your question, so here's the answer- Size 10 = 28 and 10.5 = 28.5. On the bright side, backcountry has a great return policy, so if the boots don't fit you can always exchange them.