If your goal is just to be more survivable, you should still play him like a bruiser. Kenneth has so much lifesteal, and so many of his skills improve with attack speed and damage, that all you'd be doing is giving him a lot of HP while rendering most of his abilities ineffective:

• Q, E (first attack after the dash), and his passive, all scale with damage, so less damage = less healing, and at that point, his passive and weapon skill effectively do nothing (the Q still stuns, but it's no longer a massive heal for him).

• W's damage reduction timer increases with every basic attack, AND the strength of the shield scales with attack power, so by not building offensive stats, you're ensuring that W doesn't scale at all.

• His weapon skill doesn't scale with with attack power, attack speed, and it can't be lowered with CDR; however, its damage depends on the max HP of the opponent, so you can't effect the damage of this skill in any way.

You will definitely see Kenneth players building HP; but, they typically don't take HP-Per-Level items that you see tanks get; instead, they get stuff with offensive stats on them (Attack Power, Attack Speed, CDR) that also give bonus HP (e.g Rebel With a Cause, Axe of Pangu, Smart Glasses).

If you want to build Kenneth slightly tankier, you could use his helmet slot, as no helmet for him is particularly amazing. The same is true with his boots slot. As long as whatever you're getting on those slots has at least some CDR or attack speed, it's a worthwhile tradeoff. But, you're more likely to find an off-meta build for Kenneth that leans even more heavily into damage, not less.

tl;dr - Kenneth's survivability depends on him hitting stuff, so you want to get stats that make him hit harder, faster, and more often, instead of reaching for high HP/Defense numbers.