Pour Your Heart Out - Another Rough Day

"He had another rough day today."
These are the words that I dread to hear when I walk in to pick up my son from Parents' Day Out, but they are also the words I expect to hear as well.  And I'm not sure what to do.  Because I don't know where this is coming from.
He's been in this PDO program since he was a year old.  He didn't give me trouble dropping him off, he'd nap without any trouble that first year.  He was a good kid most of the time and his teacher absolutely adored him - even when he did silly things like put rocks in his diaper on the playground.
As he's gotten older, we've had some smaller problems with him, usually at rest time.  For two years, it was always a toss up whether he'd be in the classroom asleep or out in the hall because he couldn't be silent and let the other kids nap.  Last year was filled with singing and kicking the walls, laughing and chattering away to himself.  But it was just naptime that was the problem.  The rest of the time, except for isolated incidents, he was a great kid.  His teachers still loved him.
But this year.... this year has been different.  He's gotten more defiant, throwing more fits.  Little things will set him off in yelling and screaming fits.  He hates to be told what to do and the fits will start when he's told that he needs to behave.  He licks his hands when he's angry, knowing that his teachers will tell him to go wash his hands again and knowing that he'll get upset by it.  He refuses to do things that he can, like writing his letters or saying his numbers.  We know that it's not a matter of not being able to - he'll do it when he wants to, but that is rarely in the classroom.
We're having him tested by the school system for special needs at his teachers' suggestion.  They want to make sure that we can all make sure we're giving him what he needs.  And on the one hand, I have no problem with that.  I WANT what's best for my son.  But on the other, there's the guilty part of me saying, "What am I doing wrong?"
For all that I know that no two kids are the same, we never had the same kind of problems with his older brother.  Yes, his brother had moments of defiance, moments where he wasn't behaving in school the way he should.  But they weren't as common as they are with my youngest.  So am I doing something different at home with him?  Am I not giving him the attention his brother got?  Am I giving him more?  Am I not working with him on things the same way I did with my brother?  What am I not giving him that I should?
Everyone assures me that it's not me.  And intellectually, I know that it's not.  But yesterday, when I walked through the door to hear that he'd had a bad day all day, I started to cry.  Because I feel like I'm failing him somehow.  I'm failing to prepare him in some way that I should.  And I'm failing his teachers, making their lives, and the lives of their students, more difficult because of something I'm doing or not doing.  It doesn't matter that the director, who also happens to be a friend of mine, tells me that we've got a lot of options to get him help.  I still feel guilty.
So tomorrow, I'm stopping by his doctor's office to pick up his medical records, making copies of his social security card and birth certificate and dropping them off at his PDO so they can send it to the school system and set up the testing.  And I have to hope that we can find a way to make things easier for all of us as we work all this out.