Thomas doesn't know how to do rituals. He knows how to do a "Find Harry" spell. Yes, his knowledge is that specific, as he describes it. That's not worth two refresh, for something we've seen him do all of once, and had to be thoroughly coached not to completely bollocks it up.
Nope, he can do a 'Find Anyone' spell. He uses it on the Stygian Sister, too. That's approaching worth it all on its' own, and he actually specifically states he can do several other useful things in the same way...that's just the only one he demonstrates, and thus the only one where we know what it is. I'm assuming his tricks are all Divination until it proves otherwise, but it's clearly a repertoire, and that's either Ritual or Thaumaturgy.
Common Rituals have very specific requirements (You must do X, Y, and Z, often at time A), which isn't what Thomas appears to do at all. Indeed, from description, what he does is pretty much exactly what Harry does, only he's worse at it.
An argument could be made that Common Rituals can accomplish this sort of thing...but I'm deeply skeptical of it, and even if it can, it'd certainly require high Lore...which Thomas manifestly does not have based on his knowledge base.
