If you check the 27cm Heads section on Junkyspot (
http://www.junkyspot.com) -- all the heads in the section labeled "Slim body Male heads" will fit. Some take inset eyes, some are bald and would need either rooting or a wig, and some come with rooted hair. (Note: the heads in the "Regular Obitsu Male" section do NOT FIT the slim body, they only fit the more muscular "regular" male).
Slim male with 27HD-M02N head (with dark brown rooted hair)
Slim male with 27HD-M02W head (no hair)
Slim male with 27HD-M03N head (it came bald, I rooted his special medieval priest hairstyle).
He can ALSO wear any of the 27cm GIRL heads -- if you're going for more of a young boy or teen look, or you like anime faces. You'll have to change his neck attachment -- just unscrew the wedge-shaped piece, and use the plain plastic rod underneath it, that will fit inside the girl heads.
Slim Male with 27HD-F03NC01 (female head). I personally think this is the best of the female heads to use with the slim male, it gives you a very bishie or youthful look -- though I do know folks who've used the other heads, or even the eyehole heads.
He can ALSO wear the Volks Neo-Goh heads -- if you can find one. Pale and "natural" skin tones match reasonably well with the respective pale or natural Obitsu skin times. (Don't get the "Guy" colored ones, those are darker and don't match well).
Obitsu slim male with Volks Neo-Goh 03 head. Fits perfectly.
Maybe someone else has examples with the Aries head, or eyehole heads -- I don't have any guys with those heads.
Hope that helps!