I have read that an adoptive parent (for example) can convert a minor child as the change in status is considered to the advantage of the child so it does not need his consent. This child can renounce the conversion when he reaches adulthood. But examples like this seem to revolve around converting a very small child without his consent. What about a boy of 12 who, though he may not be an adult in halachic eyes, would need to be coerced (and even knocked out) to accommodate a bris milah that he resists.
Can a parent/beit din physically force a child - who is making his contrary intentions clear via thrashing his body and yelling "no" - to undergo a bris milah for the sake of conversion?