Yut Lung is probably taken too young to have a Hong Kong/Cantonese accent in his English (and definitely not a modern way by any chance). It'll sound strange, but I don't really see him speaking Cantonese at home?? As in I hc the official language in the Lee household is probably Shanghainese. His old money dad definitely moved to Hong Kong because WWII, so it'd fit with our film trope that he was originally from Shanghai, which could also explain the American connection. His mother is more likely to speak Cantonese, dialect pending. The servants might too, with dialects/accents from various parts of Canton/South China. Yut Lung did mention his father was good to him so he must've picked up Shanghainese from dad. And then he'd mostly be using Mandarin for business and among his brothers.
I want to say he'd speak Cantonese "in private" but what "private" does he have after his mother died? He'd hold onto the language, but it became harder and harder to articulate or he'll reveal what he really wants.
A funny thing came up in my research, mandatory education was implemented in 1971, a year after Yut Lung's birth in manga timeline. That means I can't keep him in the traditional mansion with private tutors... Tiny Yut Lung in bowtie and shorts prepring for his first day of primary school... if his mother isn't killed :D damn i get sad thinking of his mom sewing/knitting things for her boy...