He's a good child and they want him to play more but if he stayed there any longer the darkness will disappear and he won't be able to return (we don't know yet what will happen as no one stayed more so he needs to leave soon than being sorrow later + he's already a scardy cat)
Nothing mentioned in his wiki for ppl staying in this darkness more than this. If I'm remembering correctly it was mentioned in the previous chapters (or the novel >< I'm sure read it tho)
Wait i'm dumb but why wont the blue dragon want to mc escape? Is it actually just because hes good?? huhuhuh T_T