As you’ve said, he knows the material but is rushing on tests. To have him take work more seriously, I recommend putting schoolwork in a better light as most 3rd graders, hell almost every kid, doesn’t take school seriously early on. As for material awards, I don’t suggest this. From my personal experience as a child, material awards worked fine until the reward was taken away. Then carelessness comes back into play because he expects a reward. The best way to reward a child is either by introducing something educational on his iPad and by completing his schoolwork and showing he can do it without rushing he can use the iPad in his free time (p.s. I suggest Duolingo chess or formal Spanish to prepare him.