Hello, congrats on the boy. The nub theory is very reliable. I'm going to attach two photo of your scan and point out what shows its a boy and then change your scan to what it would be like if it was a girl. If you also load a copy of your scan up. You will see I drew around the nub (or I tried to haha) if it was a girl, the nub would be more like this. Note all babies a different, so all angles will slightly vary, but an upwards nub is a boy, a more horizontal nub (nearly online with the spine, is a girl) But the angle of the nub determines the sex:
Here is my drawing over your scan:
http://www.mediafire.com/view/?4zk80y629170mu0
Here is my drawing over your scan:
http://www.mediafire.com/view/?4zk80y629170mu0