My little 4 yr old granddaughter has Neptune in the 1st, and also has Mercury squaring it, like your son.
I was worried when I first saw it when she was 1st born.
But now I see how it seems to be playing out, and here are a few things I noticed in her life so far, that may be attributed to it , in part.
When she was an infant, she had some strong allergic reactions to foods, from breastfeeding even. My daughter-in-law had to stop dairy products completely, because baby K had such bad reactions to it. [they did blood tests to see what her allergies were]
Anyway, she is still allergic to milk and cheese, but it is not deadly or severe, she just immediately throws up if she accidentally eats any.
So she is 'sensitive' to certain foods and also meds. They try not to use many medications because she tends to have weird reactions to simple things like Benadryl or child Tylenol.
It may not be an issue for your son, but it is something that sometimes comes up with Mercury square a 1st house Neptune=sensitivity to unknown substances
Also sensitivity to other people's moods and words. My grand baby kind of over reacts emotionally, if someone comes on too strong. She has gotten much better about it, but she takes everything to heart and she is so perceptive and intelligent, that she can read body language and unspoken things.
So that is one of the upsides of the Neptune/Mercury angular aspect. She is so perceptive and deep. She picks up on everything.
She loves to sing and dance and she walks around, no floats around, singing along with Disney Princess songs, and acting out the parts.
In fact, she LOVES to act out things with her facial expressions. I do believe she might be quite the actress as she grows up. lol
I already told my son that Mercury/Neptune square can indicate an ability to convincingly tell stories. And reminded him that he tried to tell me quite a few stories growing up, but he was terribly unconvincing. His little girl might be better at it than he was, so he needs to focus on teaching her the value of truth.
It will be great for her to develop acting skills for art's sake. But she must be taught at a young age that telling the truth is important, because that Mercury/Neptune square can tempt the native to be deceitful or to hide the truth.