I have tried him everywhere. But as a support my counter picks are champs that lock down the enemy, leona, thresh, braum, and instead of going all in on the adc (which tahm will counter by simply devouring) you hold onto your hook/snare/slow/root until the moment he devours. That CD is a long one early on and you can severely punish him if you ever see him eat a minion IMMEDIATELY jump onto the adc, he only has 2 offensive abilities essentially and pre lvl 6 he deals almost no damage (at lvl 6 is when he gains the bonus magic damage).
I play against him a lot in the same way you would a soraka really, you can't just bully down the adc anymore, he's one of the supports that in the laning phase you focus him first and then the adc, or get him to devour the adc and hook him back in. He's highly susceptible to slows and snares and really unless you have let him get fed in lane, he's really not going to be able to do much offensively late game. Now if he's full build? thats like fighting a full build garen that can also eat an ally and spin away.. And at that point idk, I haven't ever had to fight against a full build kench, only ever been the full build kench myself, and I honestly felt pretty invincible at that point. But so would any full build tank. The main thing is shutting down his laning phase, and understanding that if he's alive, chances are the guy you are trying to kill will also be alive, just like soraka, so either dish the damage evenly, or have a plan to snare him when he devours. But the biggest mistake a kench can make is devouring something he shouldn't because in that CD time you can easily kill of the enemy.
I especially like braum and leona against kench because chances are he devoured the adc to avoid ONE of your cc's, so as braum lets say you get your passive off on the adc, while you're stacking that try and get it started on kench so the moment he devours his ally you can pick up the stacks on him to lock them both down, or as leona you xineth blade onto the adc and wait to stun until he devours, then stun, and if post 6 ult, Leona and braum ult both shut him down pretty easily.