My son resisted getting himself dressed for a long time. He wanted me to help him, so I did. My husband used to tell him he could do it and they'd argue back and forth for 10 minutes before my husband (or I, very aggravated at my husband) would finally help him.
At around 4.5, he decided he could do it all himself. Before that he would sometimes put on his underwear or his pants, but never his shirt. I think that, until he had the shirt thing figured out, he wanted me to be there to assist with everything. One day, he got up before me, got himself dressed and has only asked for help occasionally in the past 10 months or so.