"Hey, kid," she wrote alongside a photo of her son posing in front of a beautiful sunset, "the past 5 years have been the BEST, hardest, most blessed years of my life. I love you so much. The world is yours, my love...I hope you take it. Happy Birthday Luca Cruz."
Earlier in the month Duff teased that she had quite the soiree in store for her little man. Though she and ex Comrie finalized their divorce last year after splitting in 2014, they planned to host a backyard barbecue with a Ghostbusters theme for the birthday boy.
"I always feel torn or guilty about something," she told the magazine. "I’m not working right now, and at first I stressed about that, like, 'Oh, I shouldn’t stay still this long.' And that’s ridiculous. I have a child; it’s totally fine to not be slammed all the time.
"Some of my friends tell me, 'God, it must be so nice, you get a break from your kid because you share him,'" she added in reference to co-parenting Luca with her ex. "I’m divorced, and it sucks. Well, it did suck for a while; now it’s just normal."