5 year old son likes to act like a baby at times

my son is fabulous and normal in many ways but what concerns me  at times he acts like a baby-  fake baby cry sounds,he likes to remind me of times when he was a baby,etc. today he told me that big boys can drink milk from their moms. obviously he's craving attention  which he gets from me  but not always with his father.who watches him while i'm at work( about 20 hrs wk)  i've talked to my mate and he tells me i baby him. how do i reserve this cross positively   any ideas