If you attend ROTC (or a service academy) on a scholarship, you will be committed to active duty and guard/reserve will not be an option until the initial ten-year commitment is up.

I would skip the scholarship programs unless he's certain he wants to be a military officer first, and also maybe a pilot second. Too much risk of not screening for flying along the way, or getting assigned helos or UAVs, neither of which will get you hired at an airline.

If he's willing though, I would recommend air national guard or USAF reserve after college. Since you apply only to specific units which interest you, you know before you commit what type of aircraft you'll be flying. Also if you wash out of UPT and still have a non-flying commitment, it will be part-time only so you can then proceed with your civilian career. If you complete training, after about two years you'll be released to part time status and again can proceed with your civilian career in parallel with your military track.

If he has the inclination and willingness for military service I cannot over-emphasize the advantage of military flight experience, including guard/reserve, when applying to major airlines.

The regionals are NOT, and never will be, a viable career option. They are a place to build flight experience and hopefully stand out...the majors then cherry-pick who they want from that pool leaving those not selected to rot in an unstable career at a company that can be bankrupted or shut down on a whim for the convenience of management. Since airline pay and QOL are solely determined by seniority, the loss of a job for a senior, older pilot is a catastrophic life event from which they will never recover.

If you go the civilian route, you need to have a plan, a timeline, and a backup plan. If you haven't moved on to the majors by a certain point (say age 32) then be FULLY prepared to quit and go get a real job before it's too late for your financial future.