Today, on an email discussion list I read, a parent presented a problem and requested suggestions for how they might make the disposal of a child's treasured toys "a cheerful event." The parent had secretly disposed of a couple of toys, and when the child asked about them, didn't now how to put a good face on it. She said she was dealing with a "sentimental kid", who was attached to their things and didn't want to only keep "his favorite 5."
Don't throw away or give away someone else's things without their willing permission. Not your husband's, and not your child's. You wouldn't want someone to do that to you. And forcing him to choose to keep only his favorite 5 is cruel.
Children have strong attachments to some of their things, and you have no way of knowing which things, or just how strong the attachment is or how much it will hurt if you toss it.
It's not that children are "sentimental" but that they make sense of their world by what is in it. They orient to their environment via the things and people in it. That's why, when they are quite young, they are disturbed by Mom getting a radically different hairstyle. It doesn't look like Mom anymore.
Their security in the world is based on things being in the right place, and things they can rely on not changing. If you're going to change something, talk to them about it first to see if they are comfortable or not with the change. If it bothers them don't do it if you don't have to, and make changes slowly and a small amount at a time if at all possible.
Unless your child has a huge amount of toys and possessions, you can surely box up and store in the garage, attic, basement, under the bed, on a shelf things he's not playing with presently. Then, if he asks for them, take them out and leave them out, perhaps putting something else away to have less to pick up. At some point, maybe when he's older or more secure, ask him if he's willing to part with any of them. If he isn't, then put them back and wait some more.