Your assumption that things would improve was reasonable - these sorts of reactions generally do resolve after a time. At this point, though, the situation does warrant professional intervention, because it's progressed beyond the point of what might be regarded as 'normal' separation anxiety. Almost always in such situations there is nothing really problematic at school; rather, the distress involves the child's absence from the parent. Arrange an evaluation with a pediatric mental health professional who can evaluate your son and plan with you and your wife how to proceed.