Unless his a psycho, I would say he loves you. Not merely as bf/gf, but he sees you as someone who is a major part of his life. Kinda like he wouldn't emotionally function properly without you. cheesy-ness aside, he probably means he would have been a different person if you werent there to change how he sees things, behave and do stuff.
So if anything happens that would make you guys fall out, he wouldn't take it lightly. If he's a passive person, he would feel lonely/ sad and take it as a lesson. If he's more aggressive he would try anything to remind you how you mean to him (be careful if that's the case). Either way, its best if you don't take this lightly too. Show him where he stands in your life (hugs/ kisses/proper talking)
Btw if you only met the guy for 3 weeks or less and he doesn't explain why he said that, no doubt his a bit needy. Better show him where he stands (talk).
Like others said, it just means that he views you as more than just a girlfriend. Even if you guys don't work out he still wants to be friends because you two have whatever in common.
I, personally, have always been uncertain about that phrase. On the one hand he's saying that you are important enough to him that he is always going to want to have you in some part of his life (girlfriend, wife, or just friend) and on the other hand it says that he thinks your important in his life but doesn't necessarily see a future being in a relationship with you, with emphasis on the necessarily (as that is only a slight possibility)
You got to be sh*tting me, if he said that, that means he's gonna always have you in his life no matter what, and that means he probably loves you. Also depends on how long you've been going out, 3 months? This guy's a needy creep. 3 years? It's about time he says something like that.
He is unsure where he wants to put you in his life but he knows he wants you somewhere in it. Its not necessarily a good phrase because if you were the love of his life this wouldn't be the phrase he'd say. His not prepared to let you go but I don't see him wanting you now as wife/girlfriend material. Men compartmentalize women and within the first few minutes know where they will put you. Its when they have spent time with you that begins to confuse them because then they become unsure. You have fun your loving your funny BUTT! Something missing they can't put a finger on. So in that case you become someone they want in their lives but they aren't sure which category to put you in filed under YOU WILL ALWAYS Be A PART of My LIFE. Indecisiveness sucks but confirms that your not THAT one for him.