You're going to have to deal with custom scripts, not AI Packages
AI packages are (usually) automatically disabled after they are recruited (have a CurrentFollowerFaction rank of 1). The best course of action would be to use custom scripts for them to heal.
Or if you want, let them use Grand Healing. I've seen my NPC do it, but only when he's almost dead so maybe not a reliable one.
Thanks Denmax, i was beginning to think this maybe the case. Although, i am going to investigate further the use of casting at a target and use an area effect attached to the spell.
I've had a look at a script from one of the mods where a follower can heal, and from my limited knowledge, it would seem that this effectively causes the follower to force cast a healing spell.
An idea that has stemmed from this, is to have a script attached to a followers damaging spells, which evaluates the players health and if below a threshold causes certain healing/support spells to be cast.