BPSO favors one child?

Anyone have experience with this? we have an infant and a toddler. My husband used to show so much love to and spend so much time with our toddler. I just realized yesterday that it has been months since he has played with his son. There has been no incentive for 1:1 time. My son now says “daddy is mean” and is also not interested in asking him for anything - even play time- because “dad will say no.” However he has verbalized our infant in his favorite child, kisses and hugs on her when he’s home, will cuddle her on the couch…. (Doesn’t play with her on her play gym though). But will go days without kissing our hugging our son. What the heck????? Why? It is breaking my heart to see their relationship dissolve. My son now cries when his dad is rude to him and my husband is crazy impatient with our son. How to navigate this?? Why is it happening? Any encouragement?