You know, the honest and best answer would be simply to ask him if he'd be OK with it. Chances are he'd feel a bit weird, but if the relationship didn't end spectacularly badly he'll probably get over it.
You're showing signs of concern about lying etc. Simply just talk to him first and get his blessing. Or, don't get his blessing and say "Hey, I think I'm going to date..." and throw it out there man. Do the right thing, say something up front, and save yourself grief on the back end.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."