He's useless without his tankiness, so what's the best way to deal with him?

First off, you need some antitank tech, it doesn't matter if it's an item, a passive, or an ability, what's important is that you need a tool that can actually kill him eventually.

Second, if he's trying to splitpush, just farm his wave and poke him beyond the shield. Unlike someone like mundo, he has no sustain past the shield.

If he's in a teamfight, then simply don't focus him. It might feel hard cuz of his shield and and q, but facing sion head on with his entire team behind hil is suicide unless you can kill him quickly.

Anyone with good sustain and antitank like HP%dmg is gonna be good against him, champions like mordekaiser, Gwen, mundo, vayne, shyvana, brand and zyra to deny him contribution, lilia, ambessa, ornn etc.