I wouldn't give him 6 months old. Even at 6 months old being underfed he would still be in the mid 100's. I would give him 3-4 months old tops, but more likely 3 than 4. A lot of things come into play with the size/weight because they could of been underfed for a long time, or fed on a quicker schedule than normal to help them get to size quicker.
I concure!