How have you been interacting with him? My chin is also a rescue, and I found the best way to bond with her was by letting her come to me on her terms rather than trying to force my love on her on my terms. I'd just open the door to her cage and put my hand in there and talk to her, allowing her to come up to me and check me out and then retreat as she pleased. I also made a habit of giving her bits of hay, pellets, chew sticks, and treats (sparingly) from my hand when I'd go by her cage, so she started to associate me with good things. She actually bonded with me fairly quickly (she was probably younger than yours when I got her, which could have been a factor in her quick bonding). It took a few weeks for her to get comfortable with me petting and scratching her. She still doesn't love being picked up, but that's pretty typical of chins generally. She will tolerate being picked up and held for short periods of time, but since she doesn't really like it, I usually only pick her up when I have to.
Just be patient and keep at it. Chins are nervous prey animals by nature, so they need to be convinced that you're not a scary predator out to do them harm. That process will be more difficult if they've had bad experiences with humans in the past, but with enough patience and consistency, he should come around.