Even if one missed the issue of Dick signing papers and references to that, the idea of them being father/son goes far beyond the legal status. Tim's adoption is even more recent and has been mentioned in several recent issues. Battle for the Cowl was basically about 3 sons fighting over their dad's legacy, with Alfred and Bruce also identifying each other as father and son, despite Alfred not legally adopting Bruce. Family not being about blood is a pretty huge thing in the Bat-verse.