Off the top of my head, no idea. I'll check Complete Warrior, Complete Champion, and Complete Divine, and get back to you. Let this answer serve as a placeholder until then.
EDIT: Ok, I'm back. I checked Warrior, Champion, and Divine, and no prestige class in any of those stacks with Paladin levels for determining the level of your bonded mount. Nor could I find such a prestige class in any of my books.
Upon checking the crystalkeep Prestige Class Index, I saw that there was a prestige class, the Knight of Holy Shielding, that progressed the special mount, and that it could be found in the August 2004 issue of Dungeon Magazine. So I pulled up my August 2004 issue of Dungeon Magazine (sometimes it scares me, how much D&D stuff I have) and gave it a look. At every even numbered level, the Knight of Holy Shielding gets a class feature that takes one of two forms- they may either advance the effective level of their bonded mount by 1, or the level of their paladin spellcasting by 1. Overall, I don't think it's a good class.
There may be a prestige class that stacks with paladin for your mount in some book I haven't checked, maybe a campaign specific one from Forgotten Realms or Eberron, but you'll have to look for it. If I haven't found an appropriate class by now, with the kind of resources I've got, you'll probably have to accept that if you want to play a multiclass pally, your pet horsey isn't going to level up with you. Sorry, bro... Err, sis.
If I were you, I'd just take Draconic Leadership from Draconomicon, and ride around on a Wyvern.
Update: I took one last look at my sourcebooks, and I found something for you: The Triadic Knight, from the Forgotten Realms book Champions of Valor. First level of the class gives you a special mount, stacks with the pally mount if you already have one; you add your paladin and triadic knight levels to determine your mount's stats. So, it's pretty much exactly what you're looking for.