I'll be candid since you're asking for feedback and I believe it's not going to help you if I sugarcoat it. I believe your intentions are good and I have some experience with this because my survivor doesn't like gifts either, so here goes:
Regarding his comparing your giving the gift to his mother, to rape, obviously that's totally over the top. He is extremely triggered, a small thing in the present ignited a truckload of dynamite from the past. The key is that it felt like a violation to him and in his mind, based on his life experiences, violation =rape.
So yes it was an overreaction on his part. But it was still a violation on your part, albeit a small one, but survivors are exceedingly sensitive to any boundary violation.
So how was this a violation? Well, he was clear about no gifts, yet you brought one anyway. He drew an unequivocal boundary, and you ignored it. In reality you probably saw giving it to his mother as sidestepping the boundary to some extent...after all, you weren't giving it directly to him. But in his mind, a gift is a gift.
So when he found out you'd crossed that boundary, an alarm went off in his head. The survivor learned: People who do not respect boundaries are not safe people. People who cross boundaries hurt me.
And you didn't tell him that you'd brought it to his mother. He only found out after the fact from her. So you were less than honest. That's major to someone with trust issues. And going to his mother might feel manipulative to him, rather than going to him directly, you covertly used other family member in your "agenda." I don't know what role his mother had in his abuse, but if she's not a safe person, then going to her would take on even more sinister overtones, tantamount to colluding with the enemy.
I'm sure you meant well, but to someone with trust issues, what you did could be totally misconstrued.
It also crossed my mind that he may have acted happy when you gave him the pillow, but he may have actually felt upset, and was playing the expected part of "grateful recipient." Running to hang it in his room, etc seems a bit over the top. Survivors are trained to hide their feelings and "perform" to please others, when they are put on the spot. Only afterward do they realize they feel angry and violated. So perhaps this could be a delayed reaction to some extent, and finding out you'd given it to his mother, was the last straw.
I wondered reading your post whether you asked first if a gift was OK, before giving it to him? If you didn't ask, and he hadn't given permission, then you actually violated his boundary twice -first giving it to his mother, and the second time giving it to him.
Re-reading your post I just picked up on that last bit when you said he was JUST starting to open himself to the possibility that you two might have a future....he probably felt extra vulnerable and at risk, as the beginning is a scary time for a survivor scared of intimacy...and the object of romantic interest also becomes a source of threat/danger. So finding out about the gift and his mother, was unfortunate timing. His mind may be grabbing onto it as "evidence" that his fear is justified.
It sounds like he's going to need time and space for his trigger to settle, and you may have to earn back his trust. He needs to know that you didn't mean to do it, but now you understand why it upset him so much, and you won't do it again. You will need to scrupulously follow all boundaries he sets, no matter how minor. If he says he needs space then leave him alone, until HE initiates contact. Otherwise it will feel intrusive.
Triggers are like landmines and we step on them all the time with our survivors. Over time we learn to dance around them. It's not fair. You and he SHOULD be able to enjoy exchanging gifts, but gifts have been warped and twisted for him, and they will probably never feel the same to him as they do for you.