Uhhhh ... a permanent ring of protection is not *that* powerful. I'd place a value on it at about 40,000 GP.
What do you really get from it?
1.) AC bonus of +2 deflection versus evil creatures,
2.) +2 resistance bonus to saves versus effects generated by evil creatures,
3.) Stops charms, possessions and domination effects,
4.) Stops most summoned creatures (though not of an alignment opposite the spell ... an evil wizard can still summon a hound archon to attack a good cleric protected by protection from evil).
1 & 2 are handy, but they are limited to certain foes. Against many foes, they're useless. Also, they tend to be redundant. People don't want a 'part time' deflection or resistance bonus, they want full time protection. As a result, they'll often seek out other sources for these bonuses. In the end, I'd rather have a +1 resistance bonus and a +1 deflection bonus that worked all the time than a set of +2 bonuses thatwork only against evil.
Charm, possession and domination are not terribly common. PCs may go many levels without seeing one of these effects. Even when they do, they get a saving throw. This protection only has an effect on the game in those rare circumstances when these effects pop up and when the saving throw is failed. In a few select campaigns, such as one where the PCs are hunting vampires, this ability is increased in value, but in most, it is a minor issue at best.
Summoned creatures are an odd issue to consider in this context. Some DMs never use them. Other DMs use them with a majority of their spellcasters. In some games, a PC may face summoned creatures four or five times during the 13 or so encounters it is supposed to require to level up. The value of this ring's third ability changes dramatically depending upon which style of game is used, but in a game where roughly one set of summoned monsters per character level is faced, this nifty ability is useful, but not overpowering. The summoned monsters are rarely more than anannoyance that makes it difficult to get to the real threat, the summoner.
All in all, a nice ability, but nothing to get too upset about.
As for denying your PC all the benefits of the ring ... just tell him to get it identified again. Then he'll find out that he was mistaken ... it only grants *part* of the benefits of a protection from evil spell.
