Dear J.,
I tend to save things. Too many, I'm sure but my 24 year old daughter certainly appreciates some of her more stellar projects still being around. Little by little I give them to her to take home to her own house for safe keeping. I'm the same way with my son who is 14. He's made some pretty awesome stuff. Find a way to put shelving in your son's room or hang things if possible.
I am a complete sentimental sap, but even I toss out the slime, goop and things like that. Yes our children made it and yes it was wonderful and fun, but there is a compromise that comes with it. My son has made 3-D things showing a cross section of the different layers of the earth, etc. We still have those. One of his history projects was so awesome he lent it for display at the school and shares it, but it has a permanent home in his room and always will.
It's not possible to display every single thing all the time, but you can certainly rotate things. As he gets older, you will certainly have to be selective about the things you save permanently, but maybe have a big rubbermaid box to store things in and at the end of each school year, go through them, let him relive making them, and find a couple of things from before that he might not care too much about saving. I tried to save at least a few things from each of my kids years in school. Be it artwork, a poem, a project. I still have little things they made for me for the holidays and bring them out.
But, like I said, never did get sentimental about slime. lol
Stuff like that, keep it for a week, let him play with it and then it's bye bye slime. Besides, he knows how to make more if the urgent need ever arises.
Best wishes!