He will seldom be equally proficient in both languages.
A person is seldom required to be "eqaually proficient" in two languages.
A simple "proficient" in English would suffice.
My own experience is the reverse of this: at age 8, my family moved to Brazil, and our training in Portuguese came from a Berlitz book on the voyage by ship from New York to Sao Paulo.
Once in Brazil, we settled in an "Interior" town where we were the only Americans, Ponta Grossa, and I went to school there.
In a short time I was sufficently "proficient" to play with other kids my age, to learn math and science, and to read along with the other kids Brazilian history, songs, etc.
I went to and from school on the city buses, and generally got along well, except when some stranger mistook me for one of those much-reviled Germans!
Needless to say, we continued to speak mostly English at home, except with the maid and the yardman; even my mother learned very passable Portuguese. My younger brother and sister, however, grew up speaking mostly Portuguese, and when we returned to the U.S. several years later were at a bit of a handicap in school - for about two months.