Every guy is different, and as there are signs for different problems, they could be mis-interpreted. I would say that if your guy is starting to distant himself, or doesn't want to do the things you two normally do together, something may be wrong. The only way you will really know is by confronting him about it. Sometimes I feel as if my bf feels a certain way and I get myself all worked up, then I talk with him about it and it turns out to be nothing. Don't draw conclusions, but also try not to be naive.
Signs--he stops calling you as frequently as he did--or messaging you--always has something else more important to do--tells you "we'll get together" and it doesn't happen--big picture--you can sense it.
Seems withdrawn....lacks interest in you and your day....stops complimenting you....doesn't want to go out anymore (or not as often)....stops saying "I love you"....are a few that I can think of.
Just come right out an ask.... This way everything is in the open. You can deal with anything. At the very least you will have your answer
the key word here is "long term"...all guys loose interest in long term anythings. Just some guys might fake it for a while better than others.
He doesnt want to have sex that often...
Lots of things, it all generally amounts to him wanting to spend less time with you.
Signs? I don't know.. Why don't you just ask him?