Arbitrary Skepticism: He has gradually become accustomed to talking with the anthropomorphic animals on the island, yet still refuses to believe in anything of paranormal or mythological origin even if it is right in front of him.
He doesn't have any problems believing in aliens and Robot Girls either.
He winds up on the other end of the spectrum, when he tries to convince the islanders on the existence of the aforementioned aliens.
He did have trouble believing in the anthropomorphic aspect of Airantou's animals, but that quickly died away when Shima-tora made a...convincing argument.
A recent chapter had him get sick and get a fever which made him start to believe in magic. Then when he was still recovering he started to mix science fact with magic, and then he freaked out when he realized what he was saying.
Justified since his family placed him under a spell that made him skeptical of anything paranormal to ensure that he wouldn't figure out that his sister had supernatural powers. As of chapter 126, this spell's effects are gone and with it his skepticism.
Groin Attack: He gets because he doesn't believe Tohno is a kappa.
Lap Pillow: He is the pillow for Machi while she is sick and vice versa.
And for Suzu because she asked him which causes a three way brawl for his lap...
Made of Iron: In the beginning he would actually have to rest a day after being beaten to a pulp.
Now he can fight all 4 guardians, fall in a river, get eaten by a fish, and still be perfectly fine the next day. He has come a VERY long way.
He may have always been this. He drifted at sea for 7 days without any food or water with no visible discomfort. He may have just been recovering from that still in the get go which explains how weak he was in the beginning.
Harem Seeker: Inverted. If her dream is to be believed, she wants one for Ikuto. As long as she is his legal wife and others are just concubines, and she gets to have his baby first, its OK for him to have other girls.