Unless the judge's son works in some capacity that directly relates to this merger, I don't see any problem. Microsoft is a giant company. The judge can hit them with the world's largest fine (there's nothing like that in play here, just saying) and it will have no noticeable impact on their son if he works anywhere else in the company.
If their son is Phil Spencer, or somebody in the executive team at Xbox or Microsoft, that's one thing. But even if their son is working in gaming somewhere in the company at a low level, you'd have to think pretty low of that judge to say they can't be fair in this case.