He runs into his playroom and immediately goes for the container. I recommend a large mat underneath so it’s easier to pick up. Mine was leftover fabric from my curtains. You might try a white shower curtain from the dollar store. They make a pretty affordable play mat.
I do wonder how dry lentils taste because he keeps trying to eat them. This is an activity that requires some supervision and a little self awareness if you think it’ll frustrate you to have stuff all over the floor. I think using beans might be easier for cleanup.