Here's a list of my suggestions.
Damage Resistance should be 10/good...only able to have their DR reduced by those of good...perhaps a celestial sent to punish the paladin for some transgression. NOT being vulnerable to those they should be strongest against.
I like the Divine Bond, but if you're going to summon a celestial horse, keep the weapon bond as well, give paladins both abilities.
And last, make Paladin Spell Casting spontaneous. There's no reason for paladins to have to memorize their limited numbers of spells.