It's been a while since I dissected the current selection of boy fashion dolls!
Fortunately, from a sewing perspective, while there are a lot of different bodies here, they aren't all that different!
Here's a breakdown of the smaller-than-Ken-doll-boy-dolls currently made by Mattel. On the smallest end, we've got Monster High, who is the only boy doll in this picture who is in 'Slimline' boy pattern size. Everybody else: Reboot Monster High, BTS, Ever After High, "Slim" ken and the new "Original" ken are all 'Medium' boy pattern size.
The picture speaks for itself: Reboot MH boys and BTS boys have itentical body sculpts, right down to the molded on details of their underwear. Both are slightly smaller but very similar in sculpt to Ever After High boys. "Original" Ken is extremely similar in sculpt and proportion, but 108% larger which is why patterns work if enlarged to 108%.
The only doll that looks different is Slim Ken, but only because he is sculpted smooth, vs with all the extra cut-aways that would give the appearance of muscles/less body fat. Remember, dolls aren't like people! Adding muscle just means that someone sculpted muscles, not that the doll worked out and gained muscle mass.
There is a little bit of variation in the Monster High Reboot/BTS/Ever After High/Slim Ken set. EAH and Slim are sliiiighly thicker.
Not enough to keep them from sharing pre-made clothes, or even make it obvious that they are not wearing clothes that were not made for them, though. They all fit, just sit a little snugger/looser depending on if you're going from the smaller MH Reboot/BTS to the slightly larger EAH/Slim ken or vice-versa.
As always, this is even less of a problem for handmade doll clothes because the velcro/snaps/etc you sew in are always fitted per the doll you're sewing it for, so you always have a perfect fit!
Final Aside: The BTS dolls aren't different enough to warrant their own review, BUT they are very nice. Hair, faceup, and body are all well done, with the body plastic feels like a denser/sturdier version of the MH Reboot body which is an improvement as that body feels kind of flimsy/cheap. The only downside is the awful plastic clothes but... hey, when you sew that is the easiest problem to fix ;)