M.O.
For my husband's 30th (pre-children), I decided we needed to shake things up a bit...I don't know where you live, but I made plans to go to the Riverboat Casino across the border in Indiana. I made it a surprise, a month before his birthday just so he wouldn't get suspicious.
It was awsome. Essentially you are planning a group outing - so you have to have about 25 people commit. I asked everyone to cover the "group admission" charge for themselves. The casino person organized to have a tour bus meet at a local bar, all of our friends came early, then a friend who was supposed to take by husband golfing arrived early and they decided to go to the local bar for a beer first. Guess who they met!
I had preordered a sheet cake, decorated the bus and had the movie "casino" playing on the bus for the ride there, along with coolers of beer & soda. The ride there was a total RIOT. We got wrist bands and meal tickets when we got to the casino. We had a great time there and we didn't have to worry about someone driving home drunk. We had a relaxing late ride home and everyone was invited to come back to continue the festivities at our house.
I felt awkward asking everyone to pay for themselves but NO ONE cared! Everyone thought it was an awsome idea, and the best part was that it worked for our parents, friends and even elderly family. I've been asked if I'm going to do the same for his 40th because everyone had such a good time.
He'll appreciate having some time that's all about HIM instead of "the family", "being Daddy", etc. Besides, you can still do something cute with your son, just don't make that the ONLY thing.
Hope this helps!