It could be to do with his sensory processing. My DD (almost 6) has just been diagnosed with
"Sensroy Processing - Tactile Defensivess"
Hers in more so in relation to clothes and the feel of things, but it may be possible that your son sufferes from sensory processing disorder.
Having said that, it is common for young kids to diskile the texture of grass, sand etc, but they usuall grow out of it.
Will he touch it with his hands ? Like if you have him a bucket of sand will he play with it?