Have you considered using a gentle spray from the kitchen faucet? I know my faucet has two different kinds of sprays. Regular and the kind you see in the shower (you know, the one that comes out in little streams around the nozzle head). If so, this is what works for my beardie. You don't want it blasting, however, as I'm assuming that would scare them quite a bit. If not, I'd try soaking him first to soften up whatever part of his feces is left on him, draining the water, washing him gently with the toothbrush, wash the sink out, and give him another soak again, just to make sure he's all nice and clean.