If you're going to call him Robbie, I really think it would be best to officially name him Robert, and call him Robbie as a nickname. Nicknames don't look good on resumes or age well. Even if you never call him Robert, I think it's important that you put a full formal name on the birth certificate so he can have something professional to go by. Robbie doesn't really age well, and everyone will think it's short for Robert.
However, on a separate, more important issue, it doesn't sound like you really want to name him Robert OR Robbie.
What if you use Robert, but use it as a middle name and find something you and your husband can agree on for the first name? It's your child too, you shouldn't be forced into naming him something you don't want to. Just tell your husband how you feel, and try to find something you can agree on that allows your son to have his own identity.
Miles Robert Gonzales
Benjamin Robert Gonzales
Owen Robert Gonzales
James Robert Gonzales
Anthony Robert Gonzales
Hudson Robert Gonzales
Robert "Robbie" Jameson Gonzales
Robert "Robbie" Oliver Gonzales
Robert "Robbie" Sebastian Gonzales
Robert "Robbie" Vincent Gonzales
Robert "Robbie" Maximilian Gonzales
Robert "Robbie" Alexander Gonzales
Hope this helps and good luck :)