like dee said it could be a sensory thing but like others said could be nothing and he might grow out of it..
i used to look after a boy with autism who had severe sensory issues and sand was something he hated too.. to get him used to it we introduced sand to other things he liked.. small amount of sand in playdough built up more and more over time so he could slowly get used to the texture.. sand in paint.. or a small amount for him to play with other toys in like cars or plastic animals.. id imagine you could do the same with grass as well... even he will now get in the sand pit and play and he loves it was just a matter of him getting used to it..