I read an article once where a mom was talking about this (can't remember where, wish I could...). I can't imagine how tough that must be to hear. The mom in the article had ended up establishing a non-physical "hug" that she and her son could share. Something like a code phrase or something, that didn't cause him physical pain but let him know how much she loved him and that she wanted to show him affection. If I remember right, she said it was the best way for her to still feel like she could give him comfort and love without hurting him. But it was still sad, and she still said she would give anything for him to want and enjoy hugs.
Maybe he'll become more comfortable with it over time and with sensory therapy?