"This whole idea of him going to USC to study business and economics is because he wanted to be an entrepreneur," Arnold told ET's Ashlan Cousteau. "He figured out that I have always taken care of my own money. I've never had a business manager and I'm the only one in entertainment that never lost any money. He wants to make sure he's falling in the same category of 'let's worry about how much money we make and also how much money we keep.' That's very smart."
"He's doing great," Arnold continued. "I'm so proud of him, but not only of my son. I'm really proud of my daughters and of my other kids. They are all really jewels. They're hard-working, and for me the most important thing is that they are passionate in what they do."
In Midnight Sun, Patrick will romance Bella Thorne's character, a 17-year-old girl with a rare photosensitive skin condition which causes her to have a potentially fatal reaction from exposure to sunlight.
"He's a very good actor," Bella told ET. "He's strong; he's interesting; he's intriguing. He's very good at this character."
While Arnold raved about Patrick, the 21-year-old actor is just one of four children that Arnold shares with his ex Maria Shriver. In an interview with Howard Stern on Wednesday, Arnold stressed that he still has a great relationship with 25-year-old Katherine, 23-year-old Christina and 17-year-old Christopher as well as Patrick.
"My kids are a straight 10," he gushed. "I am so proud of them and I'm so in love with them. I love my kids so much."
Arnold can next be seen on the big screen in Terminator Genisys, opening July 1.