What about a handmade wooden keepsake box, like this? He could keep some of his favorite mementos in there, and take it with him when he leaves home permanently. For the one in the link, you could include maybe a nice family photo? Or a favorite photo of him? (I like the family photo idea, he can look back on it many years from now with .) We got our son’s at Old Fort Niagara. Etsy has some. Or just google around. I especially love the ones that have compartments. They run $150-300.
Or what about one of those blankets made of old tee shirts? A friend of mine did one for her son for his HS graduation and it was gorgeous. She hung it up for display at his graduation party.
Yeah we got our daughter an iPad when she graduated high school and she didn’t want it . We returned it and she got her self a very expense purse…..lolWhenever someone asks here for gift suggestions, my answer is always the same: ask the receiver. I get people want to surprise the other, but I prefer to make sure it is a gift appreciated, actually used and not ends up in a drawer, returned or given to goodwill.
Ask your son what he wants. You can set some rules that is has to be something like your daughter's ring, a gift that will last a long time. Not something frivolous. It has to be similar to your daughter's to make sure she is not going to say 'if it wasn't for this tradition, I would get a computer/money/trip too?'
Let your son weigh in on it. It is his gift. Give something he really wants or needs.