Help him now before it is too late. My daughter has a problem of eating sand at the sandbox. When she has a bowel movement there is sand in it. This is probably normal behavior and experimentation simply to be supervised and discouraged. If you find that you cannot distract your daughter from eating sand and you have to make a big deal out of it, sandbox play shouldn't be allowed in this case.
Illness can develop from eating sand, especially if animals have access to it.
There is one medical condition that you should consider, however, some children with iron deficiency called Pica, which is eating dirt and other unnatural substances. Your child should probably be screened by her pediatrician to rule this out. My daughters received the Varicella vaccine, but from what I understand, it neither guarantees lifetime immunity nor that they won't still get a mild case. So far we have data for slightly more than 20 years after receipt of the vaccine. This is probably due to natural boosting by exposure to wild chicken pox in the community.