Your duck grew up, and has hormones. If he imprinted on you, he likely believes he can mate with you. Which can result in aggression.
I suggest handling less, if he runs at you, block him using a broom or a pool noodle- no hitting or kicking or anything like that- just block if you need to. Don't allow any biting, don't try and pet him
It's very possible that his hormones will settle eventually and he will calm down. It is possible that you can have a good relationship with him again, but its important that you understand that these animals have instincts and urges, and even if they love us, they can and will follow those instincts. He isn't doing it to hurt you. Time will tell whether or not he will calm down
What is their housing situation, do you have other ducks, and if so how many males/females?