potty training?

When a baby or young toddler starts taking their diaper off, it only means they are able to do that. It has nothing to do with potty training readiness. At this age it is extremely unlikely for him to be ready, that will probably take at least another year, possibly two or more.

Sure, you can make your life hard by putting him on a potty every half hour, and possibly even get ' lucky' enough to do that right when he has to pee, but that is not potty training. It's just an alternative method to catch pee.

What does he wear over his diaper? A baby who can undo diaper tabs may not be able to undo the poppers of a onesie, or to pull down a pair of pants, or to undo a pair of overalls. In my experience that's the secret to keeping a baby or young toddler in diapers: if they can't undo their clothes, they can't undo the diaper underneath the clothes.

Yes, by the way, the smallest size pullups should work for a big 12 month old. They worked for my kid. I sometimes used them when it was so hot that I let him run around without clothes on: he could get off a diaper unless I put a onesie over it, but he couldn't get out of a pullup. I think they are too expensive to use all the time though.

He's to young to start potty training.