Why does he say no? Hard to offer productive arguments without knowing what his reservations are.
Here are some basic guidelines for convincing a parent...
1. Save your money and prove that you can support your pet on your own.
2. Do your research thoroughly and prove that you know HOW to take care of your pet.
3. Find some other responsibility in the home that you can take on and show that you're able to fulfill your responsibilities consistently and well, even when you don't feel like it.
4. Have a plan in place for emergencies...saved money...a vet lined up...a friend who can watch the animal for you when you go out of town.
5. Some positive arguments for having a snake as a pet: They don't bark, they don't chew on your shoes, they don't shed fur all over the furniture, they don't eat $60 worth of dog food every month, they only have to be fed once a week, they don't need a lot of attention, they don't need obedience school, they don't need a litter box, they don't need shots.
Check craigslist for used tanks...often very cheap that way. Also, pet stores have an option called a "critter keeper" that LOOKS like a glass fish tank, but is much less expensive because they aren't made strong enough to hold water like a fish tank is.