Although your wish to have a son is natural, it is my earnest opinion that you are not justified in taking another wife solely for that reason. First of all, how can you be so sure that you will be gifted with a son, even if you were to remarry? Secondly, it is unfair on your part to do so without your wife’s consent; for perhaps, as a respectable and caring husband and Muslim, you would agree with me that you ought to honor your marriage vows. In other words, if you are coming from a culture where taking more than one wife is not the societal norm and that your wife would not have married you if she ever suspected that you would do so, then you definitely need to obtain her permission before taking a second wife. The Prophet, peace be upon him, said, “Of all the terms we enter into, there is none more worthy of compliance than the conditions we enter into while contracting marriages.” There is no reason to limit these conditions to the written ones. It is therefore only reasonable for us to suppose that they must also include all of the verbal as well the tacit agreements that are simply assumed or taken for granted in a particular culture or society unless they are in clear violation of shari’ah. The jurists also tell us that customs and norms of a society, unless otherwise contradicted by shari’ah, have the force of law.  So it would be unjust and unfair on your part to simply go ahead and get married without your wife’s consent.  You are therefore best advised to be content with Allah’s will and choice for you. You may do so by consistently meditating on the following supplication:

Allaahumma ij’al lee nafsan mut’ma’innatan tu’minu bi liqaa’ika, wa taqna’u bi ‘ata’ika wa tarda bi qada’ika wa takhshaaka haqqa khashyatika

(O Allah, grant me a soul that firmly believes in meeting You, and is content with the provision you have apportioned me, and is satisfied with Your decree, and fears You as You ought to be feared.)