My Fiance does not let me on his phone anymore, is he cheating?

I have been with my fiance for 4 years and he does not let me on his phone anymore, in our first year we were both often on each others phone and after about a year we both never had a reason to go on each others phone because we trusted each other - there would obviously be odd times where we would do a quick check of each others phone randomly but there were never any red flags. However, in the last 12 months I have found that he is on his phone pretty much 24/7 and is always messaging someone. I have asked him who he's been talking to and he always says "the boys", I know for a fact that there is a group chat so I asked my friend, whose boyfriend is also in the same group chat to check how active my fiance was. She found that the last time he opened the group chat was about a week ago and then when he is active in it it's only a few messages asking them to meet him at the basketball court. He has also recently started bringing back left over packed lunches and I asked him where he got them from and he said a coworker has been making food for his lunches. This made me feel very uncomfortable as he kept saying that I would get a long with her as we have very similar interests... I told him I was uncomfortable and would like to take a look at his phone which he refused to do. I didn't tell him that I know he's not talking to "the boys" when he says he is, but I did ask him why he didn't let me on his phone anymore as I was feeling insecure and he said that he just wants privacy and that he hasn't given me a reason to feel insecure and that it was issues within myself and that I shouldn't start something. My bestfriend (different person from the above) has told me that I should steal his phone run away to her house so that I can look into it. But I'm worried that this is all because of my own insecurities like he said and doing this will ruin our relationship if there's nothing there...
My Fiance does not let me on his phone anymore, is he cheating?
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