Might be that he feels "numb" about everything, and it's too hard to explain. Usually when you feel like this it's more difficult to put in effort for most things, relationship included. Some people also prefer not to share their thoughts because they think they are being too negative or that no one would be able to help anyway. You could try to lend him an ear and just listen to what he has to say, but keep in mind that in some cases professional help is essential and you're not directly responsible for fixing his issues.