I wonder what it would be like to be married to someone who made me feel wanted and beautiful. There are lots of reasons a husband might lose interest in sex. Fatigue and financial worries are legitimate causes for many men. But you also seem to connect his disinterest with the birth of your baby. Pregnancy and delivery of a baby cause more than just abdominal stretch marks and post-partum depression for the mother.
New dads get their own set of psychological stretch marks that can make sex less appealing.
A father often reacts to the invasion of his space by a competitor. Suddenly he doesn't have you all to himself. Here's this other little male getting time at your breast. The baby puts new time constraints on both of you. Your focus of attention and affection may have shifted toward your son.