Widgetseniority.com has lots of good info for delta specifically but also has a page that compares the seniority progression of all 3 carriers. Assuming both carriers remain about the same size as each other, It looks like UA and DL are +/- 5% throughout your son’s carrier.

DL is better til about 2030
then UA is better until 2050
then DL is better for the remainder of the chart that goes out til 2053.

AAL has a 10+% advantage over the other two for the next 30 years.

someone hired today at 30 at DL would retire around # 900