So I'm guessing that later we'll find out that there's something very important information in that box, and the mc could've prevented some future tragedy if he had just asked about the box/looked inside..? That's just my guess..
Even so he already is mentally of the age that the mother wished him of being when he's supposed to open it, so there's really no excuse for him not to find out about what's inside right now. It's about your own life and survival, so you should try to get all the information that you can. Anything that can be beneficial for your success. (That is the mental state that he should be having rn)
Also I hate it when in these sort of stories it's rarely consistent on how the mc acts and thinks. (Like being mentally adult and still acting/doing stupid kid like things)