D.D.
First, what you are going through is completely normal. The myth that children suddenly become potty trained is exactly that, a myth. You are likely going to have phases where you are going between diapers and undies, where, over time, your child will be spending more and more time in undies. It's really best to be responsive to your child's needs as to him, this is a really, really big deal. In fact, it sounds like that is exactly what you are doing.
So, you want to be supportive and encouraging to get him potty trained, not pushy. You need to see what he is ready for and remember that it is completely normal to move two steps forward and then one step back in this process. With that said, also remember that there are times when you need to think about the most practical solution for a particular situation, such as going on a plane trip. Let's look at this situation: You are in the middle of potty training, you know that it is normal to sometimes go between undies and diapers (and he doesn't mind being in diapers), you are going to be in a situation where getting him to a bathroom on time is going to be a major issue, and you are also going to be in a situation that involves increased stress on both you and your little ones (increasing the probability that there will be an accident). So, do yourself, and him, a favor and let him wear a diaper. That way you can still encourage him to use the potty, but you won't have the stress or worry of a possible accident during your trip.
One more thing. Please do not listen to his preschool teacher. I don't know if it's that she knows nothing about child development or simply does not have enough information about this particular situation, but you clearly have a much better handle on what your child really needs than she does.
