He's disappearing now.What should I do?

My boyfriend and I have been together about a half a year ; at first he was sweet and thoughtful and never was late (or kept me waiting) but as time went by he changed.He started to ignore my calls and barely repliedn to my messages;generally speaking, I could hardly find him when I needed him.I couldn't reach  him for over a week and this situation happens a lot recently. Moreover, I found a girl calling him in the morning and he just hung up the phone in front of me but he was sending her messages when he was having lunch . too many semi-colons. I saw the chatting records on his phone and it is obvious that they were flirting. However, he explained that they were just joking ,and now I don't know where he is and what he is doing .He didn't pick up the phone.It's like I'm nobody to him and I'm invisible to him.He only shows up when he wants me but he isn't here for me when I need him.I feel like I'm betrayed and left in the dark alone.What should I do?  Good but you need to be careful of your punctuation.  Also you keep changing tenses.  What should you do? Dump him before he dumps you.