Biologically speaking, it'd be a mix, plain and simple. Some features of each of the mixed ancestry groups. Maybe a thick brow, slightly upturned nose, large canines, slightly tilted eyes, long fingers, fine hair, hair in potentially unusual places, slightly pointed ears, etc. Pick a few traits and have fun.
Mechanically, it's going to be one of three things IMO: Human, Near-Human, or Mongrelfolk.
It would be human if you decided that those aspects of its heritage showed through most. I'm only 1/4 Scottish, but those seem to be many of the features that show through. These things happen.
Near human, if there were some noteworthy differences, like low-light vision or some such. Open up some odd-ball abilities as feats at first level. If the PC takes any, he's near human. Otherwise, he's human. He'll breed with others as a human would.
If you decide the child is an freakish mix that actually displays the 25% influence of elf and orc, then make it a mongrelfolk. That's pretty much exactly the explanation given for their existence.
IMC, I had a 1/4 elf PC (son of 1/2 elf PC) marry a half-orc NPC. I ruled the kids were just human. Over the generations since, they have largely married humans, with a couple of elves in the mix, so rarely does anyone manifest any orcish traits. When a scion does show orcish blood, it's usually just in the form of being strong or tempermental.