Talk to mom about how child is doing with the baby at home. If he is feeling left out and sibling rivalry type behaviours are starting at home it will be escalated by making him "go away" to daycare. It may be that it becomes an every day thing or a not at all thing. I have done some maternity leaves where the child came 5 mornings so they got the benefits of "school", and mom spent the morning with baby bath, cleaning house, napping and errands so that when child went home baby went down for a nap and mommy and child had private cuddle time.