How can I find out for sure if my boyfriend is cheating when he deletes his evidence on his cell phone?

I am pretty sure my boyfriend is cheating. I have been going through his phone while he is asleep but apparently he knows this and deletes his messages and calls. Also on the internet he always deletes his histoy,even on my computer. If he is on my computer when I go to sleep,when I get up the next morning and check the history…its been deleted. I have sent him some privite pics of myself to his cell phone but when I look at his pics they are not on there,so I assume there is another place on his phone he is hiding them,im sure with other pics he has of girls. I caught him cheating with his x wife a few years ago and I have caught him texting other girls over the years by looking at his cell phone. I guess he has got smarter and deletes his stuff now. Where could he be hiding his pics now? He has a droid phone. What can I do now to catch him cheating since he deletes his cell phone evidence? If he is this time then its over!! I have had enough! Please help!