If you and your husband are moved to give your son something, I would suggest going out to eat at his favorite restaurant as a family - a way for everyone to celebrate.
However, I agree with your statement that such an "accomplishment" was simply expected years ago. So many parents (myself included) continue showering our kids with outrageous birthday parties, over-the-top PTA events, and obnoxiously busy classroom parties that when our kids are older, what will even excite them? Rarely do kids ask for anything - it's parents who make the decision to provide. You set yourself up for disappointment the bigger the gift/party/event you organize for your child, if they don't show appreciation. And after all, they're just kids.
Less is more, IMO. My daughter's class is celebrating their "Kindergarten Graduation", and many parents are purchasing lavish gifts for their kids. WHAT? Gifts for ending Kindergarten?! While we couldn't be more proud of her learning & behavior this year, we're getting her nothing. Maybe a Popsicle from the ice cream man as he drives by.