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He is still very young and may not understand the differences in gender roles. He may enjoy dolls and the caretaking roll as he sees his parent's in. This may be especially true if he is with his mother most of the time and you every other weekend. He is imitating what he sees most often.
If you are very concerned and it is effecting his behavior or emotions then I would suggest you get him in to see a child therapit. They can help him explore what is going on and see if this is appropriate age related behavior or if there is more going on. Let me know if I can assist anymore!