there are special rules for dependent children who have unearned income. since you state he has no earned income the following applies
he must file a return or the parents can include his unearned income on their return if his unearned income is over $1150 and
1) if married he doesn't file a joint return and
2) one or both parents are alive at the end of the year and
3) he's not over 23
if he files his own return form 8615 must be included. this requires the parent(s) to complete their return first
or the parents can include his unearned income on their return and include form 8814. parents filing might result in a greater total tax than the total if parents and child each file their own return. the basic advantage of the parent(s) including the child's income is only one return
