Is there a Pathfinder equivalent to the Radiant Servant of Pelor? I am trying to convert my campaign from D&D 3.5 to pathfinder, and everyone is on board except for my Cleric player who doesn't want to lose this prestige class opportunity. Is there a similar alternative I can suggest to him?
Depends what he wants out of it, rules power or flavor.
In terms of flavor, Sarenrae is the closest deity to Pelor in portfolio (Sun, Healing).
Prestige classes, in Pathfinder, have been demoted to what they were kinda supposed to be when 3.0 added them - reflections of belonging to a quite specific in game organization. There are some good anti-undead ones like the Knights of Ozem, and there's a Dawnflower Dervish of Sarenrae in Wayfinder #1. But in general the Pathfinder way is that classes and class options (feats/archetypes/etc) have been made stronger and more interesting such that multiclassing and prestige classing are much less common because they're less necessary.
A lot of the difference here is in how Channeling Energy is very different from 3.5e's Turning, make sure and understand how it works first. Channeling is the default clerical power that does positive energy damage to undead in a burst; Turn Undead is a feat you can take on top of it to frighten them instead of harming them (similar with Command Undead). Turning is easier in PF (just a save, not otherwise scaled to HD) and channeling does damage by default, obsoleting the need for a "destroyed result..."
Just use the Radiant Servant of Pelor straight, where the Extra Greater Turning features instead grant Greater Turning (as the Sun Domain feature). It’s not likely to be a problem anyway.
For reference, the Radiant Servant of Pelor’s class features:
In short, the class is giving you one powerful feature that is limited to only a certain type of enemy, and the most dangerous enemies of that type are going to be protected against it. This is not a powerful class.
What the Radiant Servant of Pelor is, is close to “something for nothing” since it advances spellcasting and Turn Undead at every level (which, depending on your Domains, may be the only features of the Cleric class), but unfortunately that’s pretty common in both Wizards’ and Paizo’s work. Neither game is even remotely balanced to begin with, so finding “strictly superior” options like this is not hard. As noted above, the requirement of the meh Sun Domain, the very-weak Healing Domain, and the burning of Turn Undead uses to power most abilities all lead to some pretty hefty costs as it is.
Just from Complete Divine alone, Contemplative, Divine Oracle, Rainbow Servant, and Sacred Exorcist are more powerful than the Radiant Servant of Pelor. The Ur-Priest class, also from Complete Divine, is far more powerful than Radiant Servant of Pelor. The web enhancement for Complete Divine had the Dweormerkeeper, far more powerful than that. And so on. The Radiant Servant of Pelor is not bad by any means, but it’s not a super-powered prestige class.
Ultimately, though, the Cleric has the least grounds for complaint about Pathfinder, though, excepting maybe a Wizard or Sorcerer. Pathfinder, in a lot of ways, shafted mundane characters even harder than 3.5 did. Relatively speaking, the Cleric is even more powerful than he was in 3.5.
1 Pathfinder’s version of Turn Undead (available with a feat) involves a Saving Throw instead of a special turning check. If you adapt Greater Turning to use this rule, it effectively becomes undeath to death, which is a 6th-level spell (i.e. not available for many levels after the Radiant Servant of Pelor gets it). Because this gets around the HD caps on turning, this may be less appropriate than the 3.5 turning rules. There’s no reason you cannot simply use those rules, however.