Oh Heidi, 
I am sorry he is having this problem. Did he eat anything he wasn't supposed to? (I caught Bailey in the trash yesterday

 )
Here is a link I found:
http://www.petcaretips.net/constipated_dogs.html
Romeo is so little I would take him to the vets tommorow if he hasn't #2 yet. 
Keep us posted....well you know what I mean
