We are a bilingual household, but I was also an ESL preschool teacher in Japan (I taught English primarily to native Japanese kids around your DS's age.).
My kids are both (probably) gifted. They understand a considerable amount of what their papa says to them (He's the Japanese one.). Initially with my oldest, papa was the primary caregiver, so he spoke more Japanese than English. Once our roles reversed, so did DS's language abilities.
Both now speak English more than Japanese. A lot more because it's what they're more comfortable in. My DS used to be in a Montessori classroom where 20/25 of the kids spoke a second language at home and almost all of the parents said that the kids would understand what the parents were saying in that language, but would usually respond in English.
At the ESL preschool (not necessarily gifted), the kids who were immersed in English 5 days a week including after-school care became fluent the quickest. Those who came mornings only took a bit longer, but were still fluent by 5 or 6 years old. Students who came once or twice a week for 2-hour sessions were not fluent at all (but typically had not started learning English until they were 5).
That's been my experience...
