There will be more than just one thing listed, of course.

This one is based on his personality type:
. ESTPs are not typically easy to embarrass, but there are a few things that make them squirm. One thing that is particularly embarrassing to them is when they have an emotional outburst, particularly crying. Because they tend to suppress their emotions in favor of appearing cool and composed, their emotions can sometimes catch them by surprise. This nearly always makes them feel awkward. Another thing that can embarrass them is when they mean to do something nice for someone and it's taken the wrong way. For example, they buy someone a humorous gift and instead of the person laughing, they are offended by it.

. Like mentioned above, an emotional outburst would be very embarrassing to him. He hates crying and sees it as a weakness, and he relies on logic more than emotion, so having a breakdown like that where someone sees him crying embarrasses him very much.

. His quirk has downsides to it, just like any other one. Since he can create hallucinations, this can induce night terrors or hallucinations for him, which puts him into a panic or causes PTSD. It'll have him crying and shaking sometimes, and once the downside of his quirk wears off, he feels extremely embarrassed that he acted out like that.

. He does learn to get better control over himself when he does have hallucinations, but he still has night terrors sometimes, and he'll wake up crying. He still gets very much embarrassed by that.

. Something else that embarrasses him greatly is the fact that he still bedwets. Part of it could be his past, it could be his night terrors, especially since he's extremely distressed during them and can't seem to wake up from them. But another reason are his antipsychotics and antidepressants— they tend to make incontinence worse, and he already has struggled with it because of his ADHD.