Gary Miller/Filmmagic
Raha Lewis
March 13, 2014 10:35 AM

Tom Arnold beamed with pride when he told PEOPLE in January that the birth of his son inspired him to lose 89 lbs. But secretly he was gunning to lose 100 lbs. in total, bringing him down to 187 lbs., a goal he’s thrilled to have finally reached.

“The last 10 to 20 lbs. weren’t as easy as the first 10 to 20,” says the actor, 54, who was recently photographed at the SXSW festival. “I was happy to get below 200 lbs., but I still didn’t feel like I was done. And 187 lbs. was an even 100 lb. loss, so I went for it!”

His secret for shedding the last few pounds? Pure discipline. “I am very motivated by numbers and goals,” Arnold says of his workouts. “I write everything down, including the time, calories burned and distance covered.”

The only thing that sometimes trumped his exercise was daddy duty to his son Jax, who turns 1 on April 6. “If I’m on baby duty and I’m watching the monitor in the gym and he starts crying, I write down my 16 minutes or whatever and come back later to finish.”

In fact, Arnold found that working out twice a day for shorter intervals helped him lose weight faster. He changed his eating habits, too.

“I eat at least four times a day, just smaller meals,” the actor says. “I used to eat as if I was in some kind of an eating contest. I think it comes from having an addictive personality – always doing too much, whether it be drugs, alcohol or food.”

If Jax was his inspiration, his wife Ashley, 38, was his support system.

“She cooked for me and made sure I had my fat-free Greek yogurt and walnuts with a dash of cinnamon,” says Arnold. Initially, Ashley also changed her eating patterns to match his. “She doesn’t eat exactly like me anymore, but that’s cool. I’ve got this,” he says.

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Conscious of the fact that exercise “is the best antidepressant I’ve tried,” and that it too can become an addiction, Arnold watches his exercise levels. Even though he worked out 75-90 minutes a day to shed the last 10 lbs., he knows he doesn’t need to continue at that pace forever, because he’s learned “to eat so much better.”

Having had “a couple of abdominal surgeries, including a hernia,” Arnold also hired personal trainer MB Regan to help him rebuild his core three days a week. “We don’t use heavy weights,” Arnold says. “We do a lot of core work. I will never take my shirt off in public, but if it happened, I probably wouldn’t be too humiliated anymore.”

Most important to Arnold is the sustainability of his new lifestyle, for his son’s sake. “I’m 54 already,” the actor says. “I figure if my boy gets 30 years of loving attention and wisdom from his old man, he should have the self-esteem I wish I had. I ate like a selfish jerk before he was born, I ate what I wanted when I wanted because that was my thing after giving up drugs and alcohol. But the truth is, I still eat what I want when I want. I just want different things now.”

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