Hi, my DD (now year 5) is doing both, but she is Mids in BBA, so there is not much clash at this moment (well before lockdown). Both are excellent programmes with very nurturing and caring teachers, both with the focus on correct technique, performance and artistry.
BBA has an excellent online clases during the lockdown.
So, I guess it depends on timing (if he will be BBA junior or mids) and it might be slightly different for boys (as there is a boys class in Tring, whereas mixed class at BBA).