Our shy, antisocial kitten?

Now, we've had this kitten since birth (mom was a runaway that we took in) and mom is still with us. The kitten are 17 weeks old now. Mom had 5 kittens (all boys) and we have 3 left. Froggy was a bit handicapped, runt of the litter and some back problems that make his back legs move like a frog...hence the name.

The other two are fine with us touching them, picking them up, and petting them. Froggy isn't. When he was younger, I thought that his back might have been bothering him when he was held in "baby" position, so we always held him upright.

He won't come to either my husband or I, nor does he like to be petted or held. He plays just fine with his brothers, although he does tend to sleep a lot more.

Could anyone explain why one kitten raised in the same environment as the others could be so resistant to attention?

Any suggestions as to how to get him to be more sociable would be appreciated.

And yes, they're all getting fixed next week.