It has resulted in a more ruthless attitude - in and out of the ring. Fa, who won a bronze medal as an amateur at the 2010 Commonwealth Games, turned professional in February last year and has fought 13 times as a paid fighter but only now is he beginning to fulfil his promise. The junk food is gone and so has the relaxed attitude to training.
With big opportunities opening up in the heavyweight division, and a possible showdown with rival Joseph Parker looming, there is money to be made and private speech therapists do not come cheap.
Fa's 67-second knockout of Latham last weekend, broadcast live in the USA and New Zealand, was the most dramatic evidence of his talent being revealed via a different mindset. Fa, a laidback and friendly giant of 1.96m and about 120kg, has never been one to hurt his opponent when not absolutely necessary, but that appears to have changed.
Fa told the Weekend Herald a couple of days after his fight: "I was like a savage that night. The ref was screaming out 'stop', but I didn't want to stop. That could have been the first time I've had that feeling towards a person ... I walked into the ring with a real killer mentality."
Now, sitting in the living room of his home in Papakura alongside wife Talya, Fa says of the finish against Latham, which left his previously undefeated opponent out on his feet and propped up by the top rope: "The ref came up to me afterwards and said 'you've got to be careful, man. When I say stop, you stop'."
Talya says: "We've all been wanting him to have that killer instinct. Even his dad said he's never seen him like that before. He's got the talent, but he often pulls back when he knows he's hurt the other person.
"But he looked like he didn't want to stop at all."
This isn't promising for Fa's future rivals, but it is for the man himself. As a big man with a long reach and fast hands, he has all the tools needed to be successful if he applies himself and retains his new-found ruthless streak.
Will the Latham result be the watershed moment of his career? Time will tell, but it came via a significantly better performance than his last one against Australian Hunter Sam in Auckland in May, a lacklustre six rounds finishing in an unanimous decision.
"Before the Latham fight I was really thinking about [Ezra] and the help that he needs so I was really focused on that and planning to knock my opponent out. In the first round I felt he had nothing to give so I charged straight for him."
Fa, 28 and with an undefeated 13-0 professional record, is just as determined to learn all he can about autism. He has taken Ezra to sessions in nearby Pukekohe and he downloads information on his phone to listen to on the drive to training in downtown Auckland.
The knowledge is key, he and Talya say, to helping Ezra's development, and the pair are keen to raise awareness about autism.
"I noticed at the age of eight or nine months he didn't really watch people," Fa says when asked about when he first suspected Ezra was developing differently. "I know babies at that age do watch - they stare at your eyes or at your lips - but he didn't really do that.
"One of the common traits is that they can be behind with their language or fine motor skills. His main one is his language and social interactions. He is getting better - he didn't really look people in the eye but he's doing that now and he's 'babbling' a lot more. Right now we're focused on improving his language."
Turning professional was never on Fa's agenda until he saw the rapid development of his rival Parker, a man whom he appears destined to meet in the ring in a fight which has the potential to stop three nations at least. Both men were born in South Auckland. Fa has Tongan heritage while Parker has strong links to Samoa.
Neither Fa nor Parker made the New Zealand Olympic team for the 2012 Olympics in London but while Parker, now the WBO world heavyweight champion, turned professional straight away, Fa wasn't interested.
"At that stage of my life I was so fixated on the Olympics - it was something I really wanted and I didn't make it. I got down on myself and stopped for a bit.
"What brought me back was seeing Joseph Parker going up the ladder. Every time I looked he was rising up the rankings and I thought 'I could do that'."
Fa's alliance with American promoter Lou DiBella and his eagerness to rise up the WBO's rankings, is putting him on a collision course with Parker. Fa is nearing the end of his first year of a three-year deal with DiBella, and will fight again in the United States before the end of February. There is also a chance Fa will fight in New Zealand next month.
If so, it will be his 14th professional fight in less than 24 months - a case perhaps of making up for lost time.
"Right now we're on two different paths," Fa says of Parker. "He's shooting for [Anthony] Joshua [the world IBF and WBA champion] and I want [Parker's] belt. He is my fastest way there."
Fa and Parker fought four times as amateurs, with the score 2-2. A professional showdown between the pair would approach the level of David Tua v Shane Cameron domestic scrap in 2009.
"We've always been considered as rivals," Fa says of Parker. "The respect is there. We know we can't really be friends; mostly because of our fans - he has fans who don't like me, I have fans who don't like him.
"I think it's a good future fight no matter when it happens. It would be a great world title fight but even if it's not; it's Tonga v Samoa. It will be a big fight. The family would go nuts."
