Consider if the inflight usage of catheters might be an option for you both. For instance a condomcatheter is a relatively easy and non-invasive option when looking into catheterusage. Usage can be easily learned and different then when using absorbent materials, it is a lot easier to change out in private, even outside of a bathroom. Not only could he go a lot longer without changing but if the bag were to totally fill up (which can happen, esp. on a bit "longer" TA flights), you could easily empty it out into a appropriate medical waste material with him remaining in a seat and using a blanket to cover up if that feels more relaxed (not needed as far as exposure goes, the urinebag would be attached to his leg which you could also access through the bottom of the pants). You can easily empty said material into the bathroom.

Any and all other options can be very difficult if not impossible to do inside the bathroom. Very, very rarely there is one somewhat larger bathroom onboard. But again; I wouldn't gamble on it. Even if it's scheduled that way, you could run into equipment chance last minute, the bathroom being out of order etc. If need be, on most aircrafts FA's can make a bit of a privacy area around the bathroomdoor using curtains allowing space for a caregiver to help an adult using the bathroom (as that tends not to fit). But; not an option that most will prefer or feel comfortable with.

Discuss it with his doc. It might take a referal for a first time seeing as he is a male -traditional cathing comes with a higher risk in males than females, another thing why condomcatheter would be an easier option-, but very very rarely does a urologist not have workable options that can be explained and learned easily enough to be used without needing any medical help. Not only now for a TA-flight, but also for possible many more flights coming, other longer trips or even "just" when he wants to venture out on his own and isn't in the mood (or medical condition) to run the risk of him needing a change soon after leaving home but not having somebody close asap.

Also husband needs to keep drinking in flight! Some don't or reduce their drinking as they "don't want to deal with needing to pee in flight", very dumb thing to do! Because of altitude, cabin pressure etc. the human body actually uses more fluids while in flight than in regular. Read; one has to drink more in flight than what one could normally get by with.