Maybe allowing him to follow through with his mistakes and see the inevitable outcome would be a much better lesson than fighting him the entire way. Taking the hint, Mabelle withdrew her hold and stepped back, watching quietly but critically as she paid careful attention to his painfully obvious lack of understanding of how physical objects translated to a two-dimensional surface.
"See?" she whispered behind him once his paintbrush was off the canvas, finding that the best time to interrupt. "The perspective is off already. You made the skull far too small in relation to the vase and the candle..." she paused, preventing herself from taking his hand again to correct it. This wasn't a technique issue where she could make him feel the correct way to do it but a visualization failure, and she'd have to make him see what she meant to understand. Stepping over to the set up, she carefully picked up the skull with her left hand, wanting a better grip on it than her right could provide, which she used to pick up the tall candlestick instead.
"Which is larger?" she asked simply, holding them up next to each other for him to see. "Taller," she clarified.