K.P.
If your husband genuinely doesn't make a big deal out of birthdays, then ask yourself whether or not this is really about your need to acknowledge the milestone. In which case, I would get together with his mom and sister and put together a scrapbook of his "First 50 Years". Invite family and close friends to send pictures, letters, memories, whatever they would like. Put it together for him and give it to him at brunch. He'll feel acknowledged and special without being the center of attention.
When my father turned 50, he felt the same way and really didn't want a big party. Instead, my mom paid for my husband and I to fly to FL for the weekend and the three "kids" put together a DVD with music of family photos in chronological order. We had a great weekend of eating-out and reminiscing. It wasn't over-the-top and we all felt like we acknowledged his "big day" without overstepping his comfort level.