My ex-husband voluntarily retired and now he wants to lower child support since he isn't making as much money as he did when we divorced. Can he do that?

He can petition the judge to lower his child support. It will then be up to the Judge if he gets his wish or not. Probably will so don't hold your breath on this one not happening. Is he stopping work altogether and just collecting SSR?

Typically a modification to child support is merited when there is a substantial change in circumstances that would justify a revision.
The likelihood of obtaining a modification to support or what that modification may be depends on the facts of your case .It will be on the judges discretion.

Was your husband of age for retirement? If so then he may qualify for a reduction.

Yes he can do that. Child support is based on income. If his income has substantially decreased then it is highly likely that the order would be reduced. How old is he? Do you know if he plans on having a part time job during retirement?