I checked his phone after a disturbing text i received about him now what?

I've been seeing this guy who is older than me for 7months(casual dating) who says im the only girl he's with!Last week i get a text from an old coworkers friend saying quote"are you still messing with so and so if yes you need to know he's ing around"long story short checked his phone he txts his ex everyday and has numerous female numbers including that chicks, his text varied from sup to text me when youre here...so should i just let it slide afterall im not his gf, confront him or simply walk away without giving him an explanation?(if its not obvious i have major feelings for him) ....help....