He is a handsome pup, but buying him will tell the BYB one thing- our pups sell, lets breed some more! Sorry, but that is just my take on it. "Rescuing" an animal by buying it may give that individual a good home, but what about all the other dogs he breeds and sells? I also have aquariums and if i decide to rescue a poor little betta in a tiny cup- the store will just replace him with another. It is a vicious cycle really. Not to mention they must know nothing about the breed if they are touting them as rare whites, that or they are perpetuating a lie knowningly to sell the older pups. Either way, not someone I would trust, and look at how many cages they have! how many pups do they produce a year?
Personally I would wait for a rescue. Check out the local shelters often, you don't have to go through a boxer rescue to find a boxer. Go there in person as they are less likely to put purebred dogs' pictures online because they get adopted fast anyway. I have personally seen several boxers come through our local humane society. If you talk to the people that work there maybe they would even let you know when one became available for adoption.
Like Hanna said though, it is your decision, and a hard one.