الجواب حامدا و مصلیا
The questioner’s concern for the future of this adopted child is indeed praise worthy, but it will be sinful for him to give all of his wealth to this adopted child while totally depriving his actual inheritors. This should be avoided.
The questioner should follow either one of the two mentioned procedures: Either he should give him an appropriate amount of property, making him its actual owner and possessor (merely putting his name of the documents of the property will be insufficient), or make a will in the presence of witness that after his death, 1/3 or 1/6 of his legacy be given to his adopted son, which he must not be deprived of. In this way, both the adopted son and the actual inheritors will be considered.