I assume it to be a more casual way of saying it, sometimes that's what I'll say to a friend so I don't sound all formal and romantic.
But with someone I'm in a romantic relationship with? It's the same thing as "I love you" to me.
I take it as a casual verbal form of affection, just something said in passing. Without taking the time to intimately say something as strong as "I love you", "love you" is said sort of like "I like you" but less awkward sounding.
I wouldn't read too much in it, at least he said it right? When my bf said it to me I didn't really pay much attention to the wording, rather I paid attention to the meaning behind it, I mean he's a guy he doesn't think like females