It just takes time and lots of interaction with playing, scratching, talking, treats. I have found with my first two rescue boys that they quite quickly realised I wasn't actually going to hurt them but a lot more time to actually trust me enough to let me scratch their tummies and for them to sit on my lap. I would agree with giving him space and not forcing yourself on him. Let him come to you.
Right now I have a new girl who is very timid and I am going very slowly with her. Just letting her come to me. I am also finding that using peacock feathers (From Purrs shop) are working well, in that she feels more confident with a bit of space but I can play with her using the feathers.