Let me just relate my OWN experience and hope it ain't a downer for you.
Started as street-hire package driver. Signed feeder list every time it went up (inside list for all, at the time, friend/T package drivers that wanted to go to feeders), even tho the first few months, I didn't even qualify.
After 3 full yrs, got picked. Worked, covering vacations, from May-enf of Aug. Got thrown back to package til about Oct when peak runs were added. I got on just thru Christmas. Jan was thrown back to package til vacations started again in May. Same thing the next yr. That went on for the next FIVE yrs until finally got to stay in feeders but no bid job. Still covering, days, weeks, disabilities, extras, etc. for another, I think it was about 7 yrs.
Fast forward...finally got a bid run after 13 yrs with company, 10 of it in feeders off and on.
It sounds like your husband doesn't have that option, that is, to go back to package car. But it DOES sound like, according to you, he has company seniority. That's good. As has been alluded to here, he now has the option to take two P/T jobs inside. Sure, it won't make up for feeder pay but sitting at home don't either.
And as has also been alluded to here is to make himself available. Let it be known he will cover ANY job then DO it!
Either way, it could be a long time. It was for me, but I was used to some screwy days, hours while running heavy equipment before, so I handled it.
One of UPSs cute little catch phrases is: "Expect the Unexpected". Well, you may expect to be put to work when every YOU want but that ain't the way UPS plays. Expect to get screwed.
Carry on and good luck.