Almost 16 month old has been wanting less and less milk before nap/bed time lately..

For the past few days my almost 16 month old has been wanting less and less milk before his nap and bed time (which is good because I’ve been wanting to break the habit) but last night he only had 2 ounces before bed (he used to have 9) and he woke up in the middle of the night screaming for a while...

I’m wondering why you think this is? Do you think he could be weaning himself or? I’m a first time mom