I was going to say the same thing.
Also, when I encounter a stubborn piece of poo, I drain the water out of the sink and gently lift up Milly's foot with one hand and scrub with a toothbrush with the other. Sometimes it helps if you can get the toothbrush under the toenail to scrub.
Also, to help Lenny's stress-level out, you could get a washcloth wet and try to wrap it around his foot to soak it that way for a while instead of doing another whole foot bath. Then you could try to use the washcloth to wipe the poo off.