Whats the difference between High function autism, PDD-NOS and SID?

My son is 3 1/2 years old, can say about 300 words and can put 3 word sentences together. He is a very active, outgoing child..full of energy. He has some sensory issues he hates getting his hair brushed, cut, face washed, toenails/fingernails cut. He is not toilet trained..we are having difficulties there. I hear all the time about autism and the spectrum..but how do they determine the difference between a child who is high functioning autistic, a child who has Pervasive Developmental Disorder - not otherwise specified ( shows some signs of autism not enough for diagnoses) or if the child has Sensory Integration disorder...which can affect speech too.Whats the difference between High function autism, PDD-NOS and SID?
The lines are blurry at best. Sensory integration is a component of ASD's but you can have sensory issues and not have an ASD. Most kids especially at his age that show ASD features do have a sensory component. PDD.NOS kids have better social skills than those with HFA. Kids with HFA are more impaired than the PDD.NOS. Problem solving at this age would be finding something hidden or partially hidden when asked, puzzles, idk. This assessment questionaire I like alot.

http://www.childbrain.com/pddassess.html



My 2 sons currently score a 51, and 68. The 51 is my 7 year old, when he was younger this score was much higher. My 3 year old when he was dx PDD.NOS at 9 months old his score was a 49, I am hoping that now it will start improving. Where you are on the spectrum isn't static, it usually improves after the peak at age 2-3. Significant for PDD is over a 49.