A short movie like this is when you really need to pay attention to what is said and you can then start to formulate what it is about.
The movie is about the controlling Mom, "emotional incest" and how one try's to deal with it.
1) He MIGHT be depressed, or simply resolved to his miserable life (which I'll get to in a moment). He carries an obvious VERY flat affect which could mean either of the two. There are also degree's of depression such as "functional depression" so he may reside in that category.
2) The Mother gets upset that he is going out. During dinner she say's "You remind me of your Father." She says this at the beginning and the end of the movie.
3) We learn that the Dad walked out on the family.
4) Dennis shares that his Dad was an alcoholic.
5) What has probably happened over the course of a decent amount of time... the marriage was horrible, the Dad left, the Mom began relying and depending on Dennis to be the "man of the house" which eventually evolved into "emotional incest." Dennis is now the husband and the Mom continues to put these guilt trip "jabs" at Dennis to control him so he will never leave her and all loyalty belongs to her alone. The Mom is afraid of being left, alone, so she breaks Dennis down so he'll never leave.
6) What this has done, instead of making Dennis a "man" it has actually de-masculinized him.
7) In order to feel like a "man", he has pursued bodybuilding, the only thing the Mom can't... but tries, to control. His confidence is actually quite low and will do what he can to please others. He didn't drink at the restaurant with the girl, BUT, she didn't ask him to- she just asked WHY he didn't. At the party though, the host HARDLY pushed him and he caved... as well not offering any kind of fight to show off (in the hopes of being accepted). What happened though, he was de-masculinized again and mopes off.
8) What we see is actually a 6'5, 300lb, 30-something boy. This is why he goes to sleep in bed with his Mom at the end. He is still a child.