The human would indeed gain 500 years to his lifespan.
Shinigami who die are reduced to dust, and their remaining lifespan is given to the human they saved. wiki
It is very unlikely to see that human live those 500 years though. Shinigami are lazy and don't want to be bothered with the human world too much, so the human would become an eagerly desired target for the other Shinigami. If they would kill that specific human they would be able to gamble for 500 more years without having to be bothered with writing some new names.
So basically he would gain a lot of extra lifespan, but it wouldn't make him immune to the power of the Death Note. An example of this was seen with Misa Amane. Despite having gained the lifespan of two Shinigami, she still only lived a couple of years after their deaths, because the Death Note had recalculated her lifespan over several occasions during the story.
As mentioned by Madara Uchiha, humans with a lifespan of 124 years or higher can indeed not be killed directly.
You cannot kill humans at the age of 124 or over with the Death Note.
They could be killed indirectly by killing people that would result in his lifespan being recalculated, but as I mentioned Shinigami are lazy, so they wouldn't bother to do so intentionally, making him indeed semi-immortal :)