VampireKB, what the heck are you doing to the poor guy? Altering a hero's max level is easy. Go to the Advanced > Gameplay Constants menu and change Hero Maximum Level to whatever you want.
Other related issues that people have run into:
If you're primarily fighting neutral players, you'll need to make your hero able to gain experience from creeps above level 5. This can also be found in Gameplay Constants.
You'll probably want to increase the level of the hero's hero abilities so that the sum of them matches his maximum level. You can do this at each of the abilities individually in the ability tab of the object editor.