Your problems are ill founded. There is only one caste, a human being. In olden days, the caste system came into existence for allocation of appropriate jobs. Today anyone can do any job. In fact Dalits have the maximum preference in everything. So you should consider your son as a brilliant one who is willing to marry inter caste and such practices should be encouraged. When in nude, every man and woman look alike and you cannot differentiate their caste(there are exceptions man made!). All of us breathe, eat food and sleep alike. The inner mechanism of all living beings are alike. When you want blood for transfusion, do you reject a Dalit’s blood?