Personally, to me ... it most certainly could be mistaken for a bomb. Not everyone knows exactly what a bomb should look like, and with how they appear in movies, you can't really blame someone if they do mistake it for a bomb.
Now, if I remember correctly, when the police got there they knew it wasn't a bomb. They still arrested him though because it looked like one. Just like if you had a plastic sword and freaked everyone out because they thought you where carrying around a real sword in school. They would probably arrest you for that too.
I mean the way I see it is, nothing major really happened. So what if he got arrest ... big deal. He claimed he built the clock and he didn't even do that. He took it apart and put it into a case. Has this kid never seen a movie before?
If someone had done the same like that and brought it into an airport ... it would have been an even bigger incident than this lol.
Him being Muslim .. sadly also does play a roll in how believable one may think it's a bomb. It is ... racist .. ya. But if it actually was a bomb .. I don't think anyone would care racism played a part in saving lives. That person would be seen as a hero and the racism part would never be mentioned.
Edit:
As for him being invited to the white house, that is the dumbest thing I have ever heard. Why? Shouldn't the president .. be acting like a president and be dealing with more serious matters?