What is this strangers problem? ?

I began volunteering with children and young adults with autism and disabilities at the start of 2019 which I absolutely love! The other volunteers have been doing this for a while now and are quite cliquey. My last job was working in retail for two years, I'm familiar with cliquey behaviour which didn't bother me. 

What seems to be puzzling me is the behaviour of the only other male volunteer. We are both gay, I'm 28 and he is around 23, he rarely speaks to me, avoids me. The volunteers are in this Facebook group(letting everyone know if you can't volunteer that evening), anytime someone posts on it, he will reply within the hour. I posted tonight, no reply. People had posted before and after me, they all received replies. 

We all started adding each other on Facebook, he didn't bother with my request. 

He doesn't come across as a nice person in general, he has a whole attitude about him, I have no idea whether it's because he's gay and "bitchy". 

There was another incident with my friend who began volunteering with me, she works as a manager in hospitality and he popped into the restaurant one day, she served him and he completely ignored her throughout the service. She had also bumped into him walking through a supermarket a while ago and he managed to reply with a "Hi" after she greeted him, he walked by laughing with his boyfriend. I feel like he's just simply immature and ignorant, and needs to grow up and learn some common courtesy. 

Let me know your thoughts.👍