His damage needs to be amped up a bit and his sustain needs to get cut in half or have it regen at a much lower speed. Maybe treat it like Tahm's where he has to be out of combat before it starts regening? What ever the case as an assassin he shouldn't have that kind of build in sustain.
Another idea is to let the healing scale with health that he has built instead of damage he has. Reward him for building stuff like Steraks or BC among other things. As it stands it feels like a war of attrition in other lanes, use your mana and cooldowns to wittle him away only to have him dip out of sight and come back with a large chunk of that health back.
Now that I think about it I'd love to see more playstyles on his aswell, maybe using stealth aggressively. Perhaps your next attack against an enemy out of stealth marks them and when you hook them or stab them through you get bonus damage = to a ratio based on your current converted hp.