Here's the link: https://math.millennial-ai.com/ (no signup or detail collection etc. just go to the site and play)
This has four modules: addition, subtraction, multiplication and division. It has auto scaling difficulty, so initial rounds are simple(for adults) but its designed to get tougher as he gets confidence and starts putting in more correct answers.
I've tried to gamify it by putting a scoring mechanism that gives more score for tougher questions as well as for faster responses & continuous correct responses. Each round lasts 5 minutes and starts with the easiest difficulty.
I would have loved something like this back when I was preparing for entrance exams just to stay comfortable with quick calculations. Let me know if you guys have any feedback on the modules or the difficulty.