Wendy's asked you great questions. Routine issues and teething can play a *big* role in WIWO. But I agree, consistency really is the key! If you do things differently at this age, they'll really start to play around and see what will happen if . . .
I've done WIWO with my DD several times due to some accidental AP or getting back on track from illness or vacation. Just the other night I had to go back to it, as we'd been on vacation a couple weeks ago and had a bit of a crazy week upon returning, so really not consistent with bedtime at all. So, the first night of re-doing WIWO took upwards of 1.5 hrs, the second night 15 min, and the third she was back to going down without a peep. It's almost harder to do it the second time around, because you've lost some of your initial resolve, but don't give up or it won't get better. If you stay consistent and stick with it, I think he'll be back to independent sleep in no time as long as the routine works for what he needs.