I would think good appeal. Like AoPS, Beast Academy is strong on conceptual understanding and therefore minimized memorization. It's likely to take him deeper and into more complex math than other curriculum.
Most kids love the graphic novel approach (dyslexic/ probably dysgraphic DD9 loves to just read the books for fun - and she doesn't like math at all). However, some kids hate the busy presentation and prefer their math clean and straightforward, or are turned off by the talking monsters storyline. There are sample pages on the website you could use to see if the format is inherently appealing to him.
