I think he is the 4ths son. It fits the pattern with characters looking like their dad or mom.

Kiba looks like his mom.

Shino looks like his dad.

Shikamaru looks like his dad, Shika.

Ino looks like her dad, Inoshi.

Choji looks like his father.

Asuma looks like his dad(?) the 3rd Hokage. Not sure if the 4th is his dad or the other old guy. The one with glasses who sits on the council with the 3rd and the old lady.

Sasuke looks like his dad too. Or at least did.

Neji looks like his dad.

Shit, all of the Gennin's look like their parents!

The 4th and Jiraiya also resemble each other, but you have to see the right picture from the manga. Also, I believe that the 4th is the son of Tsunade and Jiraiya. I don't have any proof that I can mention here, but I am sticking to my guns on this one.