Have him fight with a bow or spells. If he (or any other companion) starts swinging with a melee weapon, chances of survival go out the window. Blame Bethesda for not giving us proper support for companions.
I expected this, so I set him to bow and spells almost immediately, but I think he keeps casting a touch range spell. If I go into melee range, he follows me right in, and if he gets something's attention away from me he takes a huge beating. I also equipped him with better weapons and armor.
Raise his skills, thus raising his attributes/health, and causing him to fight/defend himself better? Improve his equipment? Make sure he has plenty of potions?
I didn't want to unbalance the game more than necessary, so he's not overly powerful by default, but if you train his main skills to the max and give him amazing gear, he'll mow through most things in the vanilla game, at least. But you do need to train him, his skills don't improve through use like yours, so if you're not training him, you'll outstrip him as you level up, and since the baddies are levelled to YOU, not him, he won't fare well.
He's got amazing gear (weapons and armor from Balmora Expansion shops), and I've only just trained him in his first 30 skill points, I've also farmed the Telvanni alchemist for exclusive health potions for him, which did help, but he goes through them fairly quickly.
Thanks for advice. The training is a bit slow, but helps.