What can we do to speed I130 for adult child over 21 in Mexico up, if anything?

You will have to wait another 6 years (a few months longer if the son gets married); and there is nothing that could be done to shorten the process. Every year, a limited number of immigrant visas is made available to relatives of U.S. citizens and permanent residents; these visas are allocated to different categories and areas of the world. In the category "Adult children of U.S. citizens", citizens of Mexico receive immigrant visas now if the petitions for them were filed in July 1993.

No way to speed up. When you file after 21 the wait is considerable.

So does everyone but you cannot speed up the process.

All immigration questions depend heavily on the facts of each individual applicant, and there are no easy answers. The best thing you can do is visit with an attorney and go over your options.

Family petitions for citizens of Mexico and Philippines are notoriously backlogged. and if he was over 21 years at the time his US Citizen father filed for him, he was not an immediate relative of a US citizen and as such, his wait time is even longer. Depending on the level of his education and job experience, you should consult with an immigration attorney who may fashion a faster alternative in the employment visa options in both the non-immigrant and immigrant categories.