Don't buy him SD13 boy clothes, as they won't fit. Clothes made for all SD10's and SD13 girls will fit.
You'll find that the SD body is very challenging to buy for! You'll have better luck with jeans and shirts sized for SD13 girl, rather than for that size boy. The SD13 boy legs are much longer, and their feet are much bigger. Sometimes you can luck out with some Delf stuff, but in the main, it's best to stick with clothes sized for the SD body. Good luck!
One of the best places to get gorgeous clothing for the SD10 boys is from Sadol.com. They make lovely things for the SD10's too! I get my pants from them!
volks pattern book for SD volume 3 has a couple of outfits for SD10 they're mega-cute, they sort of look like..... http://www.seekshopping.com/aecimages/408470.jpg likethis outfit, only with more of a cloak, and this ruched undershirt thing. it's very cute if you like i can scan you the demo photo from the book? just PM me (cause i lose threads)
I have found that SD13 girl clothes will work on SD10 boys. But I've also noticed that with the past few Dolpas, quite a few really nice looking SD10 boy outfits have been released-but they do sell out fast!
I used to own an SD10 boy and he wore SD13 boy pants all the time. I feel that Volks stuff is a little hit or miss sometimes...I have a pair of SD13 pants that are fine for SD10 and too small for an SD13. Generally I didn't mind if they were a little baggy at the waist...I never ended up with a pair that was obviously too big anyway (like falling down, etc). If you like pants that hang lower at the hips, it's a plus anyway. When I changed to SD13, some of those pants turned out quite tight and made sitting more difficult. That doll used to wear SD13 dress shirts as well. The sleeves were a bit long, but not by very much...the cuff would hit that space between the wrist and where the thumb meets the hand. Tolerable, unless you're really going for a tailored look, or maybe even desirable, if you like that slightly long look. They could easily be rolled back as well. I just ended up trying a lot. And I sewed things as well. Besides shoes, the only thing I ever bought that was sized for an SD10 was a t-shirt. I really did just picked up whatever SD13 boy stuff as I pleased. I think a lot of it has to do with creating a style that takes advantage of what's available (like baggy style, cuffed pants, etc). If you want a crisp, tailored look, then it certainly becomes very difficult.
I've gotten a few "SD/13girl-sized" things, but those tend to be for girls and I don't want my boy wearing girly stuff. Some things that are specifically SD13 size (for girls) are really narrow in the waist and don't fit him at all, too. I've never bought specifically SD13boy stuff, although I did buy a pair of pants that swore it was SD/13girl and then was HUGELY LONG and almost slid off my girl's hips. It was a little annoying. I've considered just buying a 13 body and saving myself some trouble. When I went to the Tokyo Dolpa I thought I would buy him something nice and NO DEALERS had ANYTHING for his size. If it was boy clothing, it was all 13 and up. So frustrating. >:/ There are a couple of outfits on Dollmore that claim they are "SD boy" size. Some kind of bluish suit jacket, I think. Well, it's something. I would hope that with Volks releasing a new SD10 boy that they will begin to support his size (and gender) a little better. I don't want to dress my 10 as a little girl or even a little boy. I want some nice, smart-lookin' outfits like they have for the 13s and 17s.
For my SD10 boys, I've mostly bought custom clothes. I have some SD10 jeans from Sadol, tshirts from Mio, and I commissioned Kellyhime/LavenderKiss for some outfits as well. Mio is great for custom sizes, but she does take forever and a day to actually deliver on the goods. Sadol makes lovely things also, but it's kind of sad very few of her boy outfits are actually offered in SD10 (or SD9 boy, as she calls them) size. I haven't really found any other companies besides Volks that make clothes that provide a nice enough fit for SD10 boys for what I want, though it is true they can wear some larger sizes if you don't mind baggy styles. I have a pair of really nice Volks SD13 boy capri pants that came with a belt that look pretty cute on my SD10 boys, but it's just not their style. I'm also holding out hope that Volks will start making more outfits for them that aren't shorts or short pants. ^^; I really like this outfit that came out fairly recently, and have one on the way. I'm hoping to see more things like that in the future from them. :3
Oh yes, Luts Delf items fit my SD10 boy as well. I bought a pair of cropped pants (from a set) before and they fitted my boy. I also tried a pair of SDF jeans on my boy and it's slightly baggy at the legs (because of the length) but very cool. Also, jackets we sewed for our SD10 boy fit Delf boys too so I figure their sizes work. Since now Luts' SDF items are more geared for SD13, so Delf should be more for the SD9/10. We do carry SD10 items, but they always take longer/harder to sell because it's not the popular size. This sometimes direct us to sew more for the SD13 boys instead and give up sewing for the SD10 boys. Here's a not-so-great picture with my SD10 boy inside. He's wearing the Luts SDF jeans and a jacket from our shop:
