If you truly are committed to the long-term care of the animal (as brought up in the above posts), the way you convince him they are not "useless" is to talk about responsibility. I'm assuming you are young, so tell him it is a chance for you to be responsible for an animal that won't be a nuisance for the rest of the family. It will teach you about long term commitment to something that won't always be fun. It's a way for you to grow.

Show him the youtube videos or care sheets. Instead of saying "can I have one," discuss the husbandry requirements, and how you could provide the correct environments. Discuss where you would find food sources, how you plan on switching the feeding from live to f/t and why that is important. Show him your research.

When he sees that it is something you're committed to, he is more likely to see it as a useful experience in your development to adulthood and responsibility.