Kitten Development Question?

I am fostering a mom cat & her 4 kittens. They were found under a thorny bush and I had to be rather deliberate about catching them and bringing them to safety. Three kittens have adapted to being in my house, but one male kitten still acts very aggressive all the time. I know it's because it's scared. I have tried many tactics, like wrap it in a towel and pet it, comfort it, talk to it, and feed it by hand. I've also left it on its own to see if it would come around. Any time I've held it, or tried to get close to it, it hisses, spits, and growls, then shakes as if it's scared. I believe the litter is about 5 weeks old. They all have taken to soft food and use the litter box, including this one troubled kitten. Any input on how to tame this kitty?