It seems, that even though he has the health of a decaying Ewok, my Jedi Knights first companion - the droid T7 - loves to build up the threat level.
Is there any sort of medpac I can use on him, and if there is can I tell him to use it himself if his health gets low enough to trigger his "Woe is me, I'm dying!" beeping.
T7 is by default a tank. You may think "what, why? Jedi knight is a tank" well not exactly, the Guardian AC of JK is tankish, but the other half is not, so they added the first companion to be a tank by default. If you're a tank yourself and don't want it to get hurt, then you can disable the 'taunt' ability it has so it doesn't use it. Open your companion's ability bar, look for the taunt and disable it by (right clicking?) on the ability. The little gem on the upper left corner of the icon should turn off. As for how to heal the companion, you can buy med packs that heal both you and the companion from biochemists, check the GTN there should be plenty there.
There are so called "Med Units", that heal both you and your companion. But you have to use it yourself, your companion won't do it automatically. And as far as I know, they aren't available in shops, the only way to acquire them is to craft them using the Biochem crew skill, or trade with someone who has that skill (probably using GTM).
Another option might be to tell your companion not to use the abilities that generate threat. Or upgrade his gear to have lots of Armor and Endurance.
Using your class' out of combat recovery option (Meditate, Introspection, Seethe, Channel Hatred, etc.) will heal your companion as well as yourself.
Alternatively, I've found a faster method that you can use if you don't need to heal yourself, just the companion:
Dismissing your companions is instant. Resummoning them is 1.5 seconds.
Resummoned companions come back with full health.
(This happens naturally every time you get on / off a speeder)
I find without any specific healing skills or crew biochem to create the med units Svick mentioned, the best way to keep the little fella alive is to juggle his aggro.
I recently came across the issue myself and the main way I had to overcome it was to pop him into passive once his health hit a certain point (generally when he starts to whistle and beep about low health). He may keep some aggro for a short amount of time but eventually it will transfer to yourself where the damage is easier to handle with skills such as saber ward.
As you progress through the story you will eventually meet companions who are built for healing, I like to keep one with me at all times for when the going gets tough.
There are, eventually, six companions available for each class (and potentially a seventh). A Melee tank, a ranged tank, a melee DPS, a ranged DPS, and a healer -- and the droid that comes with your ship.
Each of them has two stances you can pick between (except the droid); for the healer, you can choose between maximizing DPS or heals. The ~ key (or a button on the UI near the companion portrait) will let you look at all the skills available. You can right click to toggle their usage on or off, or left click to have them use the skill on your target.
So, to make a long story short, you can get your companion to heal you or itself by targeting you or itself and pressing one of the heals in its bar, provided it is a healing-type. (At this point, as a Jedi Knight, that's probably just the ship droid -- which does no DPS and so is likely a bad choice.)
Otherwise, the default AI will have the healer companion heal you or itself automatically -- you'll want to keep aggro off of it as much as possible so that it doesn't waste all its heals on its squishy self.
As a Jedi Knight: T7 is a Ranged Tank. If you've gone tank, you'll probably want to turn off most of his tanking abilities and put him in DPS stance, until you can replace him with Kira (Melee DPS); if you've gone DPS, you'll want to make sure you keep his armor updated and might want to keep him around a bit longer (although I'd still turn off his grapple); you can still manually activate his taunt if you need him to pull something off of you.
You will also want to use your Meditate ability to heal both you and your companion between fights.
