Actually, both my kids (especially my daughter) seemed to be prone to chapped lips... and we also had lots of tubes in all jackets, knapsacks, etc. Although it's tempting to lick your lips when they're dry/chapped, try to discourage him from doing this as it worsens the problem
Often found when my daughter was congested and breathing through her mouth overnight, the dry lips would worsen (leading to licking lips, etc.)... a humidifier in her room helped the 'mouth breathing'.
Thanks both times not really chapped per say just skin split in the center of the Bottom lip same spot both times.
Weird
He is using chap stick and drinks a ton.
Season change can affect the lips, I don't know what part of the world you are in but if its currently switching to more of a dry season then this can be common. Just make sure he drinks plenty of WATER.
Also if he is having diarrhea then he will need to greatly increase the water intake.