Does anyone know how to revitalize a partially depleted Mr Handy? I haven't a clue how one is supposed to bring him back to 100% again...

No way to do it (at least not yet). Just keep 2000 caps handy - when the robot fals dead you can revive him to 100% by paying 2000 caps. And while it's damaged it's still 100% effective, so just ignore the healthbar and have those caps on the ready.