I had a thought about the backpack, and I kept meaning to share it, and I keep forgetting.
I suspect that backpack holds all of Solas's worldly possessions. Well, that and his pouches (which presumably carry his money, if he has any, and herbs, and maybe any little random collectibles he's gathered up--which are probably few and far between since he's apparently a wanderer, or something.
If he's an apostate, and not an ancient elf, he's probably got to stay mobile. So, he makes what clothing he can, steals or trades for others, and doesn't stay in one place for long enough to accumulate many items of any real value. He has to be self-sufficient, so herbalism and tanning are likely skills, along with hunting and cooking. He may know the best herbs to season fresh nug with, without poisoning everyone. And the herbs and roots and mushrooms that you need to eat along with that nug meat in order to stay healthy.
His prominent jawbones suggest that he doesn't eat overly much, even though his buff thighs suggest he's getting plenty of protein.
Now, he could also fit all of this and still be an ancient elf, but it's not likely. I never thought it was likely even though I liked the theory (and came up with it, for that matter). The explanation for all of this from an ancient elf would be that he has been watching from the Fade and he knows that he will be viewed as an apostate. He must therefore stay mobile to avoid capture/death, but he also doesn't have much in the first place--and he may have his own agenda.
Ancient-Solas is probably more likely to betray us than plain old Apostate-Solas. But if we befriend him, he will likely not betray us regardless. Or one should hope not.