A.K.
Alison,
I think you and your husband are wise to nip this in the bud now rather than later. Kids take naps usually until they're 3 or 4, and you don't want to be dealing with this for the next 3 years! You said it best at the end of your post: consistency. Your son needs to realize that the crib is his napping spot. Set a strict naptime routine that is similar to the bedtime one: feed him, change his diaper, read a few books in the rocking chair, sing him a song, lay him down and tell him "night-night". We've been doing this with our son since birth, and he thrives on this routine. Continue to let your child cry it out. When he wakes up after 1/2 an hour, start your timer for another 20 minutes and wait it out again. You'll be surprised at how often they fall back to sleep. Do this EVERY SINGLE DAY at the same time (am and pm). Eventually, he will realize that this is the way it's going to be! Your son's body will absolutely not let him become sleep deprived - thank goodness for our bodily functions! It's YOU that will have the hardest time with this!
Best of luck to you - don't forget that CONSISTENCY is the key to everything in parenting!!
Amy K.