Toddler hair

Ok so I’m currently pregnant with my second child but I have a 2 year old from a previous relationship and every time my son comes back from his dad, his hair is always brittle, tangled and dry. I have told his dad to brush his hair several times even told him the combos and hair products he can use but he doesn’t do it, my son’s hair is curly and short. It’s like his dad goes the whole weekend without touching it, washing it or even putting any sort of products in his hair. I don’t know what to do anymore, he also at times comes back with dry patches in his hair.