Kathy,
The crowds at Halloween are only a bit more crowded than usual, but not a lot. I classify them as "Moderate"-kind of in the middle.
I like that size of a crowd. Sure, I may not get to walk right on to a ride the second I breeze up, but the crowd size gives the parks an energy that lets you know you are somewhere special.
When it is too empty, you think something might be wrong!
One time that day where it is "crowded" is at the
Mickey's Not-So-Scary Halloween Party. The actual crowd size is small, relative to the park, but this EXTREMELY popular event sells out quickly. I think you should consider attending, even though there is an extra charge, if it is within your budget. It would certainly make his birthday extra-special!
Have a magical vacation!