It would not be overpriced even if it took a standard action to activate. A +2 to AC, no weight, penalties, OR arcane failure. Not to mention it would be a tad less breakable than your average large shield. You might even rule that it works against things it normally would not, such as a Phase Arrow or an incoporeal attack.
I would limit him to one activation per round. While the rules do not disallow multiple activations, I think it would be too much. Free actions should flow naturally. It does not feel right to have a PC thinking "I will deactivate the ring, make the four attacks I get this paticular six seconds, and then reactivate it." Where as allowing a PC to use Quick Draw (just an example) multiple times, at least his DEX mod's worth, does feel right. How else can a archer knock 2-5 arrows in one round. How about someone who wants to duplicate the knife thrower from the movie Desparado. That had a good flow to it.
If you want him to allow both the benefits of a TH weapon and a shield, I suggest Either,
Imposing a -2 to hit. Being magical, the shield gets in the way some, but not terribly.
or,
Adopt new tactics. First round charge, full power attack, then activate the ring to compensate for the -2 AC. Use a basterd sword or similar weapon so you can swich styles when beneficial. Fight defensively every other round between activations.