Entity Helps Player

Use the "when item in inventory tick" trigger for the specific item.
Procedure:

If entity health < 8
{
    spawn entity <invocation> at X Y Z
    remove 1 <item> from player inventory
}

About how to make it hostile, put this block in the mob AI section:

attacks entity <entity>

and replaces <entity> by the entity you want your minion to attack. Add this block again as many as you need different mobs to be targeted. Don't use EntityLiving as the mob will attack everything, including you.

If you want to make it follow you, you should find the block in the "target tasks" tab.

The only flaw with this method is that it'll activate at every tick if you don't heal and if you have summon items left.

I don't know yet how to make a cooldown ;-;

or just try to heal the player in the summon procedure.