Welsh actor Richard Mylan and his son, Jaco, 11, have visited Cardiff Market at closing time more than 500 times.
Jaco was diagnosed with autism at the age of four.
He likes wind farms, rugby, washing machines and the order and routine of life at the city market.
His father, who appeared in BBC drama Waterloo Road, said his biggest concern is what the future holds for his son.
“How will he fit into this world?” he asked in a BBC Wales documentary which follows the family as Jaco moves from primary to secondary school.
Richard and Jaco: Life With Autism is on BBC One Wales on Tuesday 4 April at 21:00 BST.