My bf is going astray?

I haven been with my bf since we were 14 everything had been good i love him and he loves me, but i have nothiced in our last years of high school that he is really looking at other girls he is startin to flirt and take off with "friends" a lot. My dad told me that he is young and it has always been us so he wants to try out new girls and see what they have to offer- i dont think he is sleeping with anyone but i was told that he has kissed someone. it really hurts and i love him so i dont want to end it. When we are together he is so sweet to me, and he always chooses me like if he were out with friends and i needed him he would be there no questions asked.. and when girls give me problems he is the first one to tell them off and i'm the one that he stays with and wants to be with... so thats y i'm confused y does he flirt and mess around if i'm the one he wants? n is there anything i can do? How can i catch him in the act cause i havent been able to prove it- should i stay with him?