As you probably noticed with your + HP ability having multiple levels, it is both the problem and solution.
Give the hero a dummy ability which does nothing each level. Then use the bonus health ability exploit to raise maximum health every time the ability level is increased. You may need a trigger if unlearning tomes are in your map.
The bonus health ability exploit works by abusing an error in the game with the ability having multiple levels. Since it never was meant to have multiple levels Blizzard forgot to have appropriate code for raising its level correctly so instead changing the ability level does nothing. However when the ability is removed it looks up the value for the current ability level and removes that health instead of the amount added at level 1. Abusing this exploit mechanic allows you to efficiently raise unit maximum health.
Make a bonus health unit ability with 2 levels. No unit actually persistently holds this, instead it is only used by triggers to change health. Level 1 adds X HP (where X is the amount per level you want your hero to gain). Level 2 adds 0 HP. When you learn your dummy ability give the hero the hp ability, raise it to level 2 and then immediately remove it. It winn add X HP due to level 1, remove 0 HP from raising it (as it is bugged) and then remove 0 HP again when you remove the ability (as it is bugged). The result is your hero now will have X maximum HP more than before.
A less efficient alternative is to use tomes of health (this method is self explanatory). The creation of item widgets will never be as fast as the above method so I advise against it. If you do use tomes of health you must remember to remove the item after it has been used as automatically used items are not removed automatically so can leak. Dead items (items that died due to combat damage) also leak, few people realise this.
