Can my ex decrease the amount of child support he pays because he has the 401k plan taken out of his check each month?

By legal definition if the amount of child support is based on gross income and by contribution it lowers that adjusted gross income then yes it could happen. The key here is legal. Putting money into a 401k is legal. He may have to explain it to a judge and there may be an adjustment. But there is nothing wrong with it.

Let me rephrase this a bit differently. If he his contributions to the 401(k) reduce his gross income, he can reapply in court for a modification based on the reduced gross. Whether it will be granted under the circumstances is questionable.

What he can't do is reduce his payments on his own. Only a court can adjust a support order.