New-need guidance CAPD-ADD

My 8-year old son has been diagnosed with an auditory processing disorder and ADD (without the hyperactivity component). School has always been a struggle. He performs very inconsistently but manages to "average out". My latest concern is his seeming inability to understand math. After much research, I have come across information about dyscalculia (sp?). He exhibits many of the "symptoms" but I'm hesitant to assign him yet another label. Is it possible to actually be tested for this disability? How would I go about getting him assistance with his math issues? We have a 504 in place to provide for modifications but it was not helpful last school year. I'm determined that this upcoming school year will be different. He is entering the 4th grade and despises school. He actually received a D on his report card in 3rd grade math!! I didn't even realize it was possible to almost fail math at that level. He has an outside tutor but nothing seems to be helping. I'm sorry to ramble. I am very worried and frustrated. Any help and/or guidance would be greatly appreciated.