Alexander (is this too common in comparison to Dominic?)- Not really... Dominic is quite common now, also. I like Alexander, but I don't love it.
Theodore- A very cute name, but it doesn't really go with Dominic.
Sebastian- I don't like this with your surname. Your surname ends with an N... try to avoid 1st names that end with an N.
Spencer (hubby really likes this, I can't think of any nicknames for it)- I like it, but it can be a surname.
Ideas: Conley, Callum, Finley, Andrew, Asher, Elijah, Everett, Emmett, Miles, Oliver, Oskar, Piers, Anthony, Cole, Kent, Jude, Darius, Xavier, Quincy, Travis, Grey, Zachary, Brody, Rory, Brady, Cody, Chase, Elias, Eli, Hunter, Forrest, Lucas, Marcus, Philip, Dante, Noah, Jonah, Jasper, Seth, Sterling, Rhys, Ross, Raleigh, Riley, Isaac, Caleb, Leo, Lawrence, Lewis, Casper, Cyril, Cyrus, Adam, Mitchell, Marshall, Maxwell, Neil, Noel, Ezra, Malcolm, Wesley, William nn Liam, August, Reid