It sounds like targeting more schools, especially a few with “full ride” or “full tuition” reputations, would be a good idea.
I believe that your mother could cash in the bonds with her and your son’s name on them with no impact on him - he would not have to be present or involved, but she would treat it as income at her high tax rate (pay the college or pay the tax man?).
She would cash in all of the ones you need to cash in, but again, subject to whatever tax she would have to pay on them. I do not think either you or your son would have to report anything at all, contrary to what you would need to report before they are cashed in (because either your mother or you or your son could cash them in up until they are cashed in).
https://www.treasurydirect.gov/forms/sav0022.pdf
My understanding is that the OR part is important, and lets either party cash the bond in, and it is not really “50-50” after it is cashed in.
Make sure grandma is on board with helping out, and have her pay tuition directly for him, that way it is not subject to gift tax at least.
(yes, horror stories of a parent being on the kid’s bonds and cashing them in unbeknowst to the kid)