honestly, i would not recommend a K6.1 to a 13 year old kid.
12.3 oz is hard to handle and the swingweight is already very high.

honestly, you really need to give insight on your brothers level.
but from what you said he seems intermediate for his age.
a full western grip generates topspin but not much as a semi-western.

I would highly recommend the APDC for some one who plays for spin.
As for the Metallix 10, that racket would offer too much power that he doesn't need
I would stay away from the Head radical because the radical and prestige series are known for their ridiculously lower power and high control levels.

so if you're brother is what we call a "tweener" (basically intermediate) go with the babolats.

The pure drive is known for it's forgiveness and many top pro use it still. It also has a nice weight and favorable for a "tweener" and even an advanced pro. so i would recommend the Pure drive if he's a tweener and the APDC if his game is a little more advanced and he is more spin prone.

Other suggestions similar to the two babolats are the
Dunlop Aerogel 500 Tour
Head Extreme
Prince Speedport Black

Kblade 98 (a little more advanced)

almost all these play similar to the babolats (with the exception of the APDC and the Kblade 98) and have literally identical specs

check tenniswarehouse.com and click on the selecting a racket link to learn more.