This is one reason I was sad that they removed ECL from Pathfinder. My suggestion would be to look back at the old ECL rules and those from Savage Species. They give a solid outline of how to judge the power levels of something with racial hit dice opposed to that of a party with class levels. They can still be implemented in Pathfinder and I have done it to large extent. Doing a Savage Progression like suggested in Savage Species does seem to be the way to go.
Make sure you know how you are going to rule on both the RP side (seems like you have a solid handle on that) and the rules side of things. It is important to decide how you are going to handle item slots, manufactured armor, feats and skills to prohibit and other things like this before the campaign starts. Just because he is awakened doesn't mean that he would have proper training to take the types of feats he would need. For instance it would likely be fairly difficult to teach a tiger how to use Improved Unarmed Strike. It would just be against their nature. You need to know how you will rule on these sorts of things and be sure you convey them to the player before hand so he can make a sound judgement on if he still wishes to do it with your rulings.