What indications in The Wednesday Wars by Gary Schmidt support the idea that Holling indeed will become an architect?
I'm not sure there are any indications actually given by Holling that he is is going to be an architect like his father; in fact, one of the primary conflicts in Gary Schmidt's The Wednesday Wars is between Holling's overachieving, overambitious father and his children.
Mr. Hoodhood lives for one thing: business. He owns the architectural firm of Hoodhood and Associates, and he is determined that nothing will keep his company from succeeding. He is equally determined that his son, Holling, will take over the firm when he is older. Literally everything is less important to Mr. Hoodhood than his company, and both of his children experience this.
His daughter, Heather, is forced to work in the Hoodhood office, and her father tells her that she will not be going to school because she already has a good job.
Holling is too young to be forced to work for the company; however, his father is too unconcerned and disinterested in his son's life to attend his play or his cross country meet. He is also too busy working to take Holling to Opening Day at Yankee Stadium, even with free tickets; he just does not show up to get Holling. When Holling rescues his sister from being hit by a bus and needs to go to the hospital, neither of Holling's parents is concerned enough to do anything but give their permission, over the phone, for someone else to see that he is treated at the hospital.
Every time Holling goes to his father for anything, Mr. Hoodhood dismisses the boy's complaint, concern, or request for help, saying that Holling must accept it because he does not want to lose any business because of Holling.
All that is to say that what Holling does or does not want does not matter at all to his father, and Holling shows no real interest in architecture except when Mrs. Baker takes him on a walk one afternoon to see the architecture of the neighborhood.
At the end of the story, Holling is able to tell his father, for the first time, how he really feels about his future. His father says becoming a man means getting a good job and providing for his family. "You hang on, and you play for keeps. That's how it works."
Holling thinks being a man is more than a job; "it has to do with choosing for yourself." His father is dismissive, as always, but asks Holling who he is. Holling answers:
"I don't know yet... I'll let you know."
While his father calls this "mumbo-jumbo," Holling has just asserted himself in a way that suggests he will become an architect only if that is what he chooses to become.