Most things can get numb if we do them too much. The major tip is just keep moving. You have a few different points you can be stimulated, your clit, the entrance of your pussy, your gspot, your bottom even. He should try moving between them to keep up the mental stimulus but let different bits of your body have a break.
Also just taking breaks and you having a go on him is good! Why not switch and see how crazy you can make him with your mouth and hands…
Beyond that, try out a nice, variable speed vibrator and see what effect that has. Although watch out because if you keep that in the same place too long you’ll also get numb.