Is he cheating?

So my boyfriend and I have been together for just over a year, we are also engaged, he is the love of my life and I knew it right away. It was totally like a "love at first sight" kinda thing and its been absolutely prefect for as long as weve known each other (3 years). But the other day he haf one of his old phones in the bathroom and i turnes it on and was going tp take silly pics for him to fins later but founs somthing else.. He had a dick pic on there that was from a messanger app and him and i never send pics like that, ever. When i confronted him about he got really nervous and fast talking and said he sent them to himself because he felt lonely. He used to be a pretty big man whore and he sent pics like that and he said he does it sometimes to feel less lonely, but i dont know what to think of that.