My daughter was diagnosed through the genetics (she had a borderline sweat), so the first thing I asked when they found her genes was the name of them so I could do research. I was told they were both very rare, my research found that one was extremely rare and the other was relatively well known, though not THE most common. If your son has the most common, I'd guess he's got double delta F508, but I'm sure a quick call to your CF nurse should give you that information.
As for my other kids, I was told that after her diagnosis one parent needed to be screened to make sure her two genes were on seperate alleles (one from each parent versus a RARE occasion of two from the same parent and thus NOT CF). If your son is double delta then it is clear that they are on seperate alleles and wouldn't be an issue. Once it was determined that my daughter did get one gene from myself and one from my husband, the CF center had my boys tested for those two genes to rule out any asymptomatic or atypical cases. They said it was pretty standard to do so. They also encouraged us to have nieces and nephews tested as well, but our nieces and nephews' own pediatricians said it wasn't necessary... <img src="i/expressions/face-icon-small-confused.gif" border="0">
If your other boys show no symptoms, maybe its not necessary to test them. But if your know your son's mutations it wouldn't be a costly blood test to run, they'd just look for his type.