My husband and I have shared 18 years of birthday partie. When he was 23, we too were on a shoestring budget and I felt so guilty because all I felt I could afford to do was bake him a cake, one thing I usually do every year, at his mom's house (because we were living on the family's property across the street from my mother in law in order to help save).
I bought her a small cluster of flowers from the grocery store (cost no more than $5) and I knew his favorite cologne. We had two children, so I bought one of those inexpensive small gift boxes maybe another $15-20), opened it rewrapped each piece and had each boy give him one of the pieces.
Anyhow, he cried he was so touched that 1) I was so loving to his mother 2) he had his family there.
Later he confessed that he thought our boys had made him these gifts he was about to open, his only disappointment was that it was not some handmade treasure from his children;he still appreciated the cologne and aftershave, but he thought it might be a piece of noodle jewelry or something else just as cheesy.
Point- dont fret on what you cannot do for him, count your blessings and put those on a pedastal for yourself and himself on that day.
Enjoy your time together, it is so precious.