Your player is associating player class abilities to NPCs. He needs to understand that NPCs aren't player characters, and don't follow the same rules as classes. They have different abilities. I'd tell him that if they had class abilities, they'd be doing a hell of a lot more damage.
I use FG Unity, along with Discord. I'm the GM, so I spent the $150 or whatever for the DM version of FG. Also I've spent a few hundred on books, so there are costs to consider with any VTT you decide to use. If you're wanting to fully utilize a VTT like FG or Roll20 or Foundry, you're going to...