i met a boy last year around november, we started talking through instagram and i knew one of his friends so he didn’t feel like a complete stranger. We met up about a week later and then continued to talk online. He quickly became my favourite person and i looked forward to a message from him all day everyday. I noticed around maybe feb/march he wasn’t replying as much and started leaving me on opened but i did prepare myself for this because of previous talking stages but it still hurt quite bad. We didn’t completely stop talking he would pop up every now and then and would almost always be flirting. I started asking when we going to meet again and he always told me he worked all the time, i asked him when i knew he was off work and he told me he had covid but i saw on snap he was at the pub the next day. Anyway we continued to talk, and then around april he started posting this girl on his story, he has quite a lot of mates so i didn’t think anything of it. He started posting her more often and stopped replying to me for weeks. We would be having a good convo and out of no where he would leave me on opened. Anyway it continued for months, ignoring me, messaging me, flirting with me, ignoring me again. He also told me he was deleting snapchat and told me to start talking on instagram, he didn’t delete his snapchat and continued posting on his story and snapping people, and then randomly removed me one day a few weeks after. Last month i got quite drunk and just admitted to him that i really liked him, i miss him and i hate it when he doesn’t talk to me, he tried change the subject which didn’t work and then replied things like “i appreciate that”.  I found out a few days ago that he has a girlfriend, the same girl he had been posting. I feel awful, i genuinely thought they were friends and believed he was a really nice boy that wouldn’t flirt with other girls whilst dating someone. It all made sense i feel so stupid that i didn’t realise sooner, maybe subconsciously i knew but didn’t want to believe it? i don’t know if i should tell him that i know, really i just want to know why. I still don’t want to lose him, i know that sounds really bad but he was there for me a lot and i feel like if i asked about it he would block me straight away. what do i do

TLDR: My favourite person has had a girlfriend the whole time he’s been flirting with me and didn’t tell me. should i tell him i know