After the war, Aphrodite blessed every demigod, Greek or Roman, with a tattoo in a random place of their body of the symbol of their soulmate's parent. 
Nico, being in the infirmary, didn't know this, so when he woke up with a sun tattoo on his neck, he was shocked to say the least. Will, running  in with a skull tattoo in the same place as Nico's sun one when the young boy screamed, noticed the symbol on Nico's neck.
After laughing for a bit and explaining what Aphrodite had done, Nico cocked his head to the side and asked "So, this means that my soulmate is someone from the Apollo . . ." and stopped short when he saw the marking on Will's neck. 
Will chuckled and said "Yeah. And I think-" he placed his hand on Nico's, leading him back to the bed. "we have a match."