How do I catch my cheating boyfriend red-handed?

My boyfriend had been flirting with other women via sms and online dating. I caught him out four times. He apologized and told me he loved me and doesn’t want to loose me, but he flirts because he is bored at night!!??? I found an sms of another women on his cell saying that she should be alone after work when all the lights are out. What do you make of that…? How can I catch him? He is now deleting all messages off his cellphone – it is so obvious. Please advise.

Jesse