Were the motor skills assessments done recently (i.e., was he almost 5 when it was done)? I ask because some of that stuff does seem to me to be pretty behind for an older 4 year old if he has had significant interest/exposure/opportunity. DD has had a number of gross/fine motor skills that she is behind in because of a lack of interest, which doesn't worry me at all, but it sounds like that is not the issue with your son. If, for example, he is trying and practicing and wanting to be able to jump from a short height without falling, but cannot do it, it seems to me that there is very likely a physical issue to blame.
Wrt the handwriting, I am inclined to think that being frustrated by an inability to write is more or less just a gifted preschooler thing, and not necessarily indicative of a problem. It's just so common that the desire to write comes before the ability to write in our kids.
It seems to me that you've already got some solid indication that he's going to need more than a standard K curriculum. How much more is something you don't know, and probably something you can't know. You could try out a math placement test (e.g., Singapore), at home if you want. It won't help with school advocacy, but it might give you a better idea of where he's at right now.
