The man says he worked two full-time jobs for years to afford expert care for his boy to be able to keep him in the mainstream school system rather than send him to a "special needs" school.
"With our love and encouragement, instead of medication and treatments, our boy has improved dramatically over the few years, becoming a footy player, a soloist in the school choir…" he wrote.
Everything has been going well but things took a dark turn last weekend when the family took a trip to the beach in Cronulla, south of Sydney.
"When my boy walked into the park with his mum to wait for us to arrive, he saw not only every kid in his actual class, but every kid in his whole grade was there, celebrating the birthday of a popular kid in my boy's class… everyone had been invited except my son," he wrote.
The man said he saw his son hang his head as he realised what had happened.
"To make it worse, some of the parents saw my boy and our family nearby and they tried to avoid eye contact and looked away in awkward shame."
The man says he calmly called his son aside and told him: "**** em. We got you and you got us. That's all you'll ever need."
Despite the dad's encouraging words, the incident seems to have affected the boy.
"But I know I walked away knowing that I didn't solve anything, my boy is more embarrassed and self-doubting now than ever, other kids and parents are treating my kid like a leper…"
The man's post received a number of encouraging comments and advice.
"You have to be honest with yourself and accept that your child is different. I did the same with my kid but then after checking out the special needs school I realised I was sending him to normal school while ignoring that he actually needs constant specialist help," a father commented.
"The new school is amazing and they work on his personality issues as well as his education. Oh, and he gets invited to birthdays now," he added.