Send him to read the Paladin forums here, or Maintankadin (Maintankadin • Index page) (Specifically the Maintankadin Basic Training FAQ that reads "ZOMG! My gear has no spellpower!")
If he comes out of that still saying he needs to gear for spellpower or int, find another tank.
3.0 changed everything for prot paladins. All plate tanks wear the same type of gear (save for obvious things like DKs going for dodge/parry instead of shield block).
3.1 made mana a non-issue for all paladin tanks (it never really was for a MT with Blessing of Sanctuary). Now even an OT can keep a stream of mana coming in so long as they're hitting something.
Seal of the Pantheon (or Reppelling Charge from Naxx) is a good trinket to give you more gearing options. I know that I found it hard to specialize my gear into Block, Dodge and Stamina sets while staying uncrittable in Naxx-10 gear until I got the Repelling Charge. But you are correct in that once he can get 540 without the trinket, he should look at better options like the Valor Medal, Essence of Gossamer, Lavanthor's Talisman, etc. Once you start stacking Str and BV, 9-12k shield slams crits are not uncommon, and threat should not be an issue.