although your DS reaction is greater than my boys was - just letting you know both my boys hated grass and my youngest hates sand as well. I didn't do anything in particular just kept going to grassy areas or sandy areas where there were playgrounds and fun things. Never really forced them to sit on it - just used to let them walk around etc and with the odd fall over etc they eventually got used to it.

(I still hate sitting on grass with bare legs cause of the itchy thing)