Dr. Jennifer Arnold shared this sweet photo of son William Rijin, 31⁄2, on Twitter Saturday, which shows the brand-new older brother deliberately pointing to a pink fish card.
"Can't believe how fast my Will is growing up!!!" the reality star Tweeted along with the image of her son on the floor.
Arnold, 39, and husband Bill Klein, 38 – who star in the TLC reality series The Little Couple – adopted Will from China this spring. Like his parents, he has a form of dwarfism.
Bringing Will home fulfilled a long-held dream of parenthood for the couple, who struggled with infertility for years.
"I walk into a room and see him and I still can't believe he's ours," Klein told PEOPLE shortly after he joined their family.
Just last month, the couple joyfully shared the news that they expanded their family again when they picked up 2-year-old daughter Zoey in India, who also has a form of dwarfism.
"Our hearts have grown in the past six months as new parents to William, and we are excited and overjoyed to welcome Zoey to our family and watch Will becoming a loving big brother," the couple told PEOPLE.