Age regression but not the typical stuff. It being the result of stress and anxiety, and whenever it happens Megumi goes quiet. He doesn't understand anything and goes completely nonverbal. And very sensitive. Gojo being the only one he's used to, nanami and shoko maybe. Each time it happens, He'd go silent and just stare at one thing and gojo would know. He'd want a stuffed toy to hold, have a simple meal and just sleep in gojos bed specifically. For the first few times they tried making him talk, but he never did. So it just became like that. But then yuji and nobara came along which made it complicated. The first time it happened, they told gojo hes being weird and not talking to them. And when gojo arrives at the scene, he finds a very frustrated megumi who walks up to him the moment he arrives and hugs him then cries in his chest. Nobara and yuji being absolutely weirded out by his behaviour and attacking them with questions. Their normal tone pretty much sounds like yelling which throws megumi more on the edge. So gojo dismisses them and takes megumi to his room where they change his uniform and hes finally hes in bed with his toy and favourite blanket. Maybe a pacifier.

But gojo isn't so nice so maybe he waits all week for him to regress to take advantage of him. Cuz its more fun when he looks confused and terrified, and can't even talk to say no. Cant fight back either. And the next day megumi wakes up clueless of what happened because he never remembers anything.