He may still need his SOWP afterall to maintain status. Once he receives AOR, then you may withdraw. But what I usually advise people in such situation is not to withdraw. They should just continue with the WP. But if it expires after AOR then you go for BOWP. If he gets approved before the WP expires, IRCC automatically cancels the WP. But
@Hyper1013 is right in the event you want to save the application money. But as my friend would say, an immigration consultant will charge more than that so there is not need to bother about the little sum. No need to withdraw.