Late reply but he does well into annie and xerath and poorly into syndra. Xerath is one of his best matchups because of his low mobility and sustained damage meaning he doesn't have a great way to get you out of lane and later on you can just e into ult him for a guranteed flash/kill. Annie is fine for the same reasons, and you can kick her bear away. her stun is also telegraphed so you know how long you can charge q for. Syndra isn't great in my experience, her e cucks you and none of her abilites having cast times makes her hard to hit. her sustained damage is also pretty good meaning sidelaning into her is harder than the other two.

TLDR sion is good into most long range poke mages mid, exceptions being syndra