If you want to talk about a guy who you think is cute or you have a crush on, do not tell him that he's "niedlich".
Also if you are telling another friend "Er ist so niedlich.", the friend might get the wrong impression.
If you say about a man that he's "niedlich" it means that you think he's kind of cute in a way or in the way he acts, but you don't really see him as a "man". The reason for this is that the word "niedlich" mostly is used to describe things that are smaller and/or younger than yourself, such as small and cute pets or small children. So if you call a man niedlich it kind of means that you think he's cute but you feel somewhat superior and are not interested in a relationship with him. You might here a girl saying to one of her friend "Er ist ja irgendwie ganz niedlich, aber einfach etwas zu kindisch für mich.".
If you say that someone is "süß" it definetly means that you ARE interested in him!!! So never tell a female friend you think here boyfriend is "süß"! She might get the impression you're interested in her boyfriend. You could well say that the way he treats her is "süß", but never that you think he as a guy is "süß". When you say a guy is "süß" it means that you like the whole way he looks and acts cute and nicely.
By the way, if you ever want to tell such a thing to a guy directly, better just say that you like him or that you think he's nice or cool or whatever. But most guys don't like either being called "süß" or "niedlich" ;)