I would screw tha passive altogether...
His current passive is the exact definition of invisible (and unfun) power.
Basically, he has an aura (which is already an invisible/unfun part), of stats (which is invisible/unfun). To top it off, the stats is a passive stats. If you get +10 AD, you hit, you deal 10 damage more, it's simple and somewhat shows an effect.
If you stand near heimer for 10 seconds, you healed 312 hp instead of 242... That doesn't make much sparks to my eyes...
The only good part about it is repairing towers, but that requires clunky playstyle and is even more of an invisible power than the rest of the innate.