The most i could see sanji doing to Kaido is cooking a meal for him to go along with his sake.
But seriously, not even gear 4 Luffy could hurt kaido, and kaido's boro breath is stronger than any of sanji's flames. his speed, observation haki and raid suit invisibility won't even matter to kaido, since sanji lacks the strength and power to even hurt or leave a mark on kaido.
Sanji would be lucky to just burn kaido's moustache I think, and even then kaido would have to be gravely wounded and on his last legs for sanji to even have that chance, like when akainu burned off half of whitebeard's moustache near the end of the battle, along with half of whitebeard's face, don't think sanji's flames is strong enough to do that though.
in all seriousness i think sanji can only do 2 things to kaido:
1. burn his moustache (akainu was able to do that to whitebeard, although i don't know how strong a dragon's follicles are, i assume they are fire resistant since he's a dragon and breathes fire from his mouth and never burns himself.)
2. inflict pain on kaido by attacking an already open wound. Not even gear 4 Luffy could injure kaido, if even gear 4's physical strength alone isn't enough, i don't see any of sanji's kicks being able to do anything to kaido. Blunt attacks seem to do little to nothing to kaido unless you are like ridiculously strong, which sanji is clearly not. Sanji doesn't have cutting attacks either, the only type of attacks that we have seen to be able to leave visible injuries on kaido thus far. Sanji's flames will also be useless too since they aren't even hot enough. But sanji attacking an already open wound could cause kaido to cry out in pain.
But yeah i think that's all sanji can do against kaido.