What should I do about my feeling of injustice towards my first wife and my son, and my desire to return to her, given my fear of being unjust to my current wife, for whom I do not reciprocate the same feelings of love?

It is advisable to fear Allah Almighty, as He says: "And whoever fears Allah - He will make for him a way out" and "And whoever fears Allah - He will make for him ease in his affair." One should seek Allah's guidance through istikhara concerning the matter. If it is possible to combine between two wives, that is better. Otherwise, one should choose the one with good character and religious devotion, for the best provision a man can be given in this world after faith is a righteous wife.