I'd skip the dinner, since it's hard to time a restaurant meal so you can get out in time. And, as you say, they will have food at the dance. You don't need a little kid with a belly ache - don't overstuff him because the dancing can get them riled up.
Are you sure he wants something like a token or a flower? Sometimes putting too much adult expectation on a child's dance can pressure them too much. They don't really know what to expect, do they? My experience with dances is that kids are pretty uncomfortable with them up through middle school and often going into high school. If the parents start outdoing each other, kids can feel left out if their parent didn't do as much. And I really hope the school has made allowances for kids who don't have moms or dads! This should not be about exclusion, but about inclusion!
Also find out if he knows how to dance or is worried about it! You could practice ahead of time. Kids can feel as awkward about dancing at this age as well as much later, just as they feel awkward about asking someone to dance, or about how to dress or act.
I'd go simpler - find out as much as you can ahead of time, and don't overdo it. Let him pick out a special shirt if you want to (but not a suit unless the school is requiring that - which I hope they are not due to the expense for families who can't afford it!), and maybe get a picture frame (or make one together) and put in a photo of the two of you either at the dance or at home before you leave.
If you haven't started a scrapbook for him, you can start one now, for milestones like this. Don't expect him to get as much out of this as you do - he still is so young and has no frame of reference, but later in life it will be fun to look back at these photos.
Remember that an evening dance is very hard on kids who are tired too, so don't build it up so much that things get ruined if he's cranky, tired, or wants to go home! Keep your expectations very moderate and make sure you are in line with what he has been told at school or with discussions with his friends. His idea of what this evening is may be quite different from yours, and I imagine that applies to all the kids!