If someone has the feat Spring Attack and he has the capacity to fly, does he still need to learn Flyby Attack, since it's almost the same thing?
Spring Attack does not specify movement mode
Unlike Ride-by Attack and Fly-by Attack, Spring Attack is generic and works for any movement mode, including flight. It says only that you may move before and after the attack. So if your goal is to move, attack, and move again, while flying, you do not need Fly-by Attack; Spring Attack will let you do that.
On the other hand, Spring Attack is not, and does not count as, Fly-by Attack for prerequisites and the like. If your goal is to get something that requires Fly-by Attack, you have to take it separately. For that matter...
Note that Fly-by Attack and Spring Attack do not do exactly the same thing
Fly-by Attack allows you to take any standard action in the middle of your movement, while Spring Attack allows only a single attack, but Spring Attack protects you against attacks of opportunity made by the target of that attack, while Fly-by Attack does not. Generally, the Fly-by version is much better. So if you wanted to take advantage of its superior benefit, you would have to take Fly-by Attack separately from Spring Attack.
That said, just houserule this mess
I do strongly recommend a houserule here:
Yes, that is correct. Although the two feats are thematically similar, they are not the same and so must each be taken individually. Pretty much all WotC-era D&D has a high focus on mechanics and the flavour of each ability is not really important from a mechanics standpoint.