So rather than take his money he'd saved to put towards the tank, his father could've done something like taught him how to invest it, and put it towards something like post-secondary education in the future. Then, once the money has made more money, perhaps his father would see that he can be responsible for the cash and consider letting him get another tank.
Now, as for other options, well.. I'll tell you what I'm doing to get my 55 gallon in August.
My grandparents have been very insistent that I live with them while I'm in university, mostly because they live all of ten minutes from campus, but I also lived with them til I was 5 years old, and I think they kinda want that time back.. plus they're getting older and may pass on within the next ten years or so, so they want to spend as much time with me as they can. Completely understandable.
Now, the awesome part is they are not charging me rent, or anything like that. And they're buying me a car to get back and forth from school.
And since they're so lenient about my fish in the first place, I was going to tell them that I'd sell both my 15g and my 10g, keep 4 of my seven bettas [the crowntails], and then after working all summer I'd use my extra cash to pay for the new 55g. And on top of that, I'd pay them for the difference in the utility bills from water changes, electricity and such.
So since they want me to live there so badly, I'm going to say 'Wellll, I come with a 55g tank, so..' and see what they say. I've casually mentioned it, but we haven't seriously discussed it.
The best thing that you can do is show them how serious you are about wanting the tank and taking care of it. It's going to be hard for your parents to understand Multiple Tank Syndrome, or the reason why most fish keepers always want a bigger tank, regardless of how large the one they have now is. That's something only us crazy fish nerds understand.

But if you can show them you'll be responsible in caring for a new tank and such, they may be more accepting of it.
But if you do have a 90 gallon, what's wrong with that one?

I'd kill for a 90..
--OFF TOPIC EDIT--
you know, i look at the birthdates of all these people i consider near-experts and Lord, if it doesn't make me feel old...
Like who, pica?