He missed Fern. He missed his equally great friend Cheryl. He missed when the Hospital-folk were all together and usually basically made slightly more sense than they currently did, and most of all he still missed his mother having any layers to spend with him. He was still a tad too young to fully appreciate the gravity of the potential end to all things as we know them, but he was at least much more upset about it than he had been letting on, especially now that the rotten Flesh Parliament guys were to blame for this boring, stupid chore he'd been suckered into.
His footfalls had become agitated little stomps, and he huffed and puffed all the while; a repetitive little "toot" from his arteries that you may have naively found quite funny, but would have chilled the exospines of the glimmering's hairiest shamblewracks.
See, maybe some of you have forgotten what exactly Willis was, or maybe you just got here, or maybe you were never really paying attention, but it may be pertinent to elucidate that he is the offspring of the head surgeon of the hospital, which isn't so much like being the son of a mother with a job as it is like being a leaf on a tree in a forest.
Everything originally of The Hospital loves and supports Willis, but these are not entirely the only reasons they are so delicate in their interactions with him, and it is perhaps also worth mentioning that what little restrictions they place on his wandering have never necessarily been for his safety.