We are simple. If guy say that you are beautiful, he means that you are beautiful. You girls always search for hidden agenda and I don't know what.
He said that you are beautiful for him. Nothing more. He likes how you look and probably act. That's all. That is good basis for relationship, but seriusly. From one word, nobody can tell you if he is hitting on you or not.
I don't think it means that he is into you. I call all of my friends beautiful because I know what's what girls want, they want to be complimented all the time. If I am flirting I tend to call them gorgeous.
Does it say it to every girl or have you never seen him say it before? If he says it to everybody, he is probably not flirting with you and just being nice. If he rarely says it or never does, then it is flirting with you.
A guy probably wouldn't call you "beautiful" if he weren't interested, because it could be easily misinterpreted. If he just wanted to be friends, he wouldn't call you beautiful, for fear of sending the wrong message. So he's obviously comfortable with you thinking/knowing that he has eyes for you. Yes, he is flirting, in my opinion. Good luck! :)