Some kittens take naturally to the bottle, but a lot don't and they have a strong preference for mom. He seems a bit weak, and as he puts on weight and gains strength he should nurse harder. If you're still keeping him with mom, make sure to weigh him a few times right before he nurses from her and right after. If he continually doesn't gain weight with her, you may have to consider separating them. Keeping him with mom is great for companionship and comfort, but it will make getting him to take to the bottle harder. You also will need to monitor his bowel movements, which won't be possible if mom is cleaning up after him. The likelihood of her milk drying up is high to begin with with one kitten, and especially now since he doesn't seem to be stimulating her much.