It's not a scam, per se, but it's not a solution either. Electrolysis is not a new concept, we ran several experiments using the process back in 7th grade basic science.
An alternator is spinning all the time a car's running, you should be able to harvest any of the electricity produced that isn't being used by the car's electrical system and run the car more efficiently.
However, there's no free lunch, and no perpetual motion machines. The electricity required to seperate water isn't free, nor does it appear magically from thin air.
There's no doubt that this guy knows all this, so his claims are somewhat deceptive.
While an alternator may be spinning all the time, it produces significant "drag" on the engine, depending on the electrical load. So basically, it takes more fuel to turn the engine. IE, no free lunch. I would call it a "scam" because it looks like it violates a few laws of physics and chemistry.
Back in the Wright brothers day, scientists didn't think flying was impossible, of course. Neither did they think that breaking the sound barrier was impossible. Now, violating the laws of physics.. impossible.