Sleepy kitten

Jessie is supposedly 6 months old; if he is indeed that old, he is very small, weighing in about 4 pounds. I suppose he may have used what calories he had staying alive. This is our one week anniversary with him and he is slowly beginning to trust a little. When I first picked him up a week ago, he would stiffen his front legs and hold his paws at a 90 degree angle as I lifted him. I wasn't sure if this was a defensive posture or a cat babinski reflex, if they even have that reflex. After a week, that seems to be going away, and I don't know whether to be sad or happy about it- perhaps both are appropriate. I really think he had learned to defend himself - what must this tiny feline scrap have been through at the hands of other people? But it's also good that he is learning he doesn't have to do it anymore.
Last evening, he escaped from the gated area we keep him in and came trotting out into the living room. He soon realized that there was no gate between him and the dogs and other cats, so his visit was short-lived, but he did OK. This morning he had nose-to-nose contact with Sam (no gate in the way) and nobody hissed; we are making progress!
I have noticed that Jessie doesn't seem to groom himself and I wonder about that. I read that kittens who are rescued at a very young age- while still drinking only milk- need to be socialized by an older cat while young so they can learn how to be a proper cat. For instance, they are taught to play fight without actually biting their playmate. Again, I wonder if Jessie's lack of grooming behavior has something to do with his past. It will be interesting to see if he starts bathing himself if he sees the other cats doing it. He has lots to learn- but all good things.