[GScivic7]

The big difference from an ex to a dx is that on the dx the manifold and cat are all one piece where on the ex the manifold and cat are two piece. If he just wants an exhaust he won't have to worry about that though.

For exhaust recommendations, and I don't know if these companies makes exhausts for 7th gens, but I recommend:

Apex'i - World Sport
Greddy - Evo, SP
Thermal R&D - Classic
RS*R - ExMag