I think allergy wasn't the right word to use for it but the easiest way to explain. He's not aro, he was 'traumatised' when he was young. Traumatised is a strong word to use but when physical symptoms appear in reaction to a certain behaviour then it's pretty serious. Seeing his mother acting lovely to his father while cheating on him made him very distrustful to people. Therefore, he only has negative reactions to people proclaiming to like/love him. Especially since he was very young when it happened.
But because he trusted Shougo(tho I did think that was a bit fast) he didn't have a bad reaction. THe same way he didn't react when Akio reacted. (sorry if I'm misunderstanding your comment) But I don't believe aromantic people are 'allergic' to love. 'a' in the first syllable usually means absence or lack. Not wanting something does not mean allergic(you can want something but still be allergic) so I don't think you should worry about that.
yahh i think if it was more along the lines of romance repulsed (w physical manifestations) instead of an allergy it would make a tad bit more sense. but allergy makes it easier for the story i guess lmao. i think it was a bit fast too but thats manga for you XD and yeah you're right aros arent allergic to love - we can be aro just because or from trauma or whatever reason. but people who cant feel romantic attraction can still want a romantic relationship. the character probably isn't really aro but it kinda struck a chord or similarity and rubbed me the wrong way with how it was portrayed but im not too worried over it~ it's just rare for stories to even come close to like... aromantic representation so when it happens its either like some character who's a player or was traumatized or something ╮(￣0￣)╭
it was cute and funny!!! buuut for some reason the whole allergy to love thing didnt really sit right in my little aromantic heart....