I'd suggest that you put some rubber gloves on, wrap Toby in a towel so he can't struggle too much and very gently wet his back end - shower nozzle at minimum water, lukewarm. If you get the poo all wet it should start to disintegrate. Watch out for bits of grass that are part in and part out of his bowels. Don't pull anything out, just clean him up as much as possible. If the poo has dried and caked into the fur you may have to do this a couple of times.

Mucky pup!
Sgurr