I'm worried that your baby is in some kind of pain. I would really take him to the vet and have this area looked at....it could only be anal glands but regardless, his reaction seems like a pain reaction to me.
Also, are you saying he is a yr old, but you've only had him recently? Bc if that's the case, it makes me wonder what happened to him in that area prior to you having him
. Wonder if his bottom got nicked by a groomer or something, ya know?
Try to slowly desensitize him too...start by petting him on his back, near the top of his tail, and only slightly move your hand a little closer to his bottom each time....in other words, you're trying to teach him that touching that area isn't always going to = pain or something.