I have a 10 YO who has asperger, (how did you get him diagnosed that young? They tell me they can't give a full diagnosis on asperger until they hit puberity!!!), ADHA, ODD, and depression. I think a lot of the problem is sensoroy since must autistic children have a sensory issue somewhere along the line. We have a food texture issue.
Does he like to swim? Play under a sprinkler? Have water gun fights?
If he can do the other things with no problem I would say it is because when he was really little he got water up his nose, or got water/soap in his eyes and it burned. Baby shampoo burns like fire if it gets in your eyes.
I would try the water gun fights and the sprinkler in the yard to help him break some of the fear.
P. R