As a partner in Wachtell, Lipton, he makes many orders of magnitude more money than the Solicitor General of the US makes. If he is offered and takes the job it will not be because of the money. Also you don't make partner in that firm unless you are a very good attorney. This would be much more of a merit appointment rather than Nepotism.
Agree he would take a big pay cut, although as an engineer, I know it wouldn’t be “many orders of magnitude” less (maybe one order of magnitude). Don’t think merit is a consideration when determining if nepotism obtains. He would be #3 at the DOJ, and his wife, as counselor to the POTUS, would be in a position of supervision & influence over him.