Warning! This article contains spoilers for The Hidden Oracle.
I apologize for my boyfriend.'
'Could you not–'
'Would you prefer special guy? Or significant other?
'Significant annoyance, in your case.”
- —Will and Nico, in The Hidden Oracle
The Blood of Olympus
When Nico arrives at the camp near Thalia's tree he is surprised to see Will planning on stalling the Roman camp. He told Nico he was doing this to get fresh air after helping Mellie give birth to her baby and held Nico's hand to show how he was still nervous about helping her give birth, which "sent an electric current" down Nico's spine to his shock and he withdrew his hand. Will immediately notices Nico's condition and nags him on not using shadow traveling anymore on his orders as a doctor. The two continue to bicker through their mission to at least make sure the Roman onagers would not fire on Camp Half Blood.
When Nico stated that he was leaving both Camp Jupiter and Camp Half-Blood, Will became upset and demanded a reason and berated him for thinking he didn't belong, saying that Nico has a lot of friends or at least people who'd like to be his friends at camp.
Another blatant hint of a possible attraction is Will asking for his company in the infirmary. When Nico confessed his feelings to Percy (but gradually left all feelings for Percy since he was with Annabeth and was not his type anymore), he walked away towards Will. This can be seen as symbolic of Nico ultimately choosing Will over Percy at the end. After meeting Will outside the camp, Nico remembers his arrival with Apollo at the camp years before and how Thalia had referred to him as hot. Nico didn't understand why he'd suddenly be thinking of that moment, but it was likely an indication of his attraction to Will, as the latter is a son of Apollo, sharing many of his divine father's traits. Nico thought of the moment when he first met Apollo, Will's father, and how Thalia had called him "hot". He wonders about the similarities between Apollo and his children.
The Hidden Oracle
At some point during the ensuing months, Nico finally comes out and confesses to Will Solace about having a strong crush on the latter, and the two begin dating. In order to sit together with his new boyfriend during meals, Nico claims to Chiron that he somewhat loses control of his infernal powers while sitting alone, and so he is therefore given permission to sit at the Apollo's Cabin's table instead, as Will produced a handy fake doctor's note, and Chiron decided it wasn't worth arguing about.
Solangelo was largely popular after the events of The Blood of Olympus. Will and Nico's interaction remained minimal throughout the events of the series before the final book. After the slight implications of Nico possibly liking Will and Riordan confirming that Nico is queer, Solangelo has been getting more attention. Although, some fans still cling to Percico over Solangelo. Many Solangelo fanfictions includes fics about what happened in the three days of rest of the infirmary.
After Rick's confirmation about Solangelo on his Twitter, fans were excited and overjoyed and Solangelo has been confirmed as canon and has been getting more and more attention since the release of The Hidden Oracle.