It may be because he is gay.
Why does my boyfriend never get a boner when we make out or I set on his lap?
People do just get boners that easily. You have to do something that will really turn him on!
He could be suffering from depression, or anything that prevents him from being himself. Symptoms of depression could be the following:
1.) Feelings of helplessness and hopelessness
2.) Loss of interest in daily activities
3.) Appetite or weight changes
4.) Sleep changes
5.) Anger or irritability
6.) Loss of energy
7.) Self-loathing
8.) Reckless behavior
9.) Concentration problems
10.) Unexplained aches and pains
Now, there are other illnesses, such as anxiety, that have similar symptoms. These aren't limited to only depression. Have a talk with him and see what's wrong, because there's clearly something, or a lot of things, bugging him.