If you want that next child is a son then you and your husband must read the name of Allah
Ya Mutakabbiro......... 10 times before sex, while sitting on the bed. The only condition is that the bed must be clean. Not only you will have a son but he will also be very salih muslim.
Wudu is not necessary.
Please note that this is not a wazifa. We dont give wazifas to have a boy as our Holy Prophet (sallal laho alai hi wasallam) had no son.