The former Lizzie McGuire star welcomed her first son with husband Mike Comrie in March.
In a Twitter post, Duff discussed her son's progress, writing that the four-and-a-half month old baby is sporting his first tooth.
"Luca is cutting a tooth!!! It's the cutest/saddest (because it hurts) getting so big my boy!! Gunna have a little toof lol (sic)," Duff tweeted.
In June, the 24-year-old was sued by a car crash victim who claimed Duff struck her with her Range Rover.
Earlier this year, Duff announced plans to release a new studio album.