Has anyone heard of any kind of leak-proof "underpants" I could put over the diaper to further prevent leaks?

If I were you, I'd use either Abena Abri-Form X-plus diapers with Dignity Booster pads stuck to the inner liner..Use between two and three...OR..you can use Tranquility ATN Briefs with Dignity Booster pads...I usually use the Tranquility Slimlines with three Booster pads in the briefs during the night...This has helped me GREATLY stop leaking...BUT...If you are worried about leakage..I'd use Bed Pads underneath him, on the bed...to prevent leakage onto the sheets...

you could use tena liners inside the pull ups-especially the super overnight ones.

I really don't know much about this but my Mom wears pull-up type

I reiterate my prior answer...If I were you, I'd use either Abena Abri-Form X-plus diapers with Dignity Booster pads stuck to the inner liner..Use between two and three...OR..you can use Tranquility ATN Briefs with Dignity Booster pads...I usually use the Tranquility Slimlines with three Booster pads in the briefs during the night...This has helped me GREATLY stop leaking...BUT...If you are worried about leakage..I'd use Bed Pads underneath him, on the bed...to prevent leakage onto the sheets...

By the way, Depends..ok...I will just say it...Depends SUCK!!! they just don't "suck up" enough moisture! I would start using Abena Abri form X Plus diapers from Drugstore online and make sure to use at least three booster pads instead of the one. I'd put the booster pads in as follows: first, lay the first pad down right in the middle, Then, lay the second one in, aligning the top of the pad (Where the padding part ends NOT the extra material) with the end of the padding of the diaper in the front and in the back. The front & back pads should slightly overlapp some in the middle. Trust me..I have been doing this for years! This seems to work. You can sometimes even use up to six pads, at night if need be and during long car trips...when you see the sign and it says :next rest area, 289 miles..lol!

BTW, Don't mind my new image....I just thought that it would be kind of humorous and light hearted to put my big head on a Pampers box since I AM a "Diaper Expert" ...The Irony.....

You've gotten good advice on products to try. Let me tell you how I made the bed for my most incontinent client. This man would be wet from his stomach to his thighs almost nightly, no matter what. I started by putting two 'Chux' pads on the mattress, then the mattress pad, with two more Taped these down with packing tape. Then a fitted sheet with two Chux pads on it, topped by another fitted sheet, and, yes, more pads. That way, in the night, while he was up tinkling, I could remove the top-most layer and we could get back to bed fairly quickly. He routinely wet the bed once per night. This worked amazingly well. Taping the pads to the sheets kept them from bunching, causing discomfort, and from sliding, causing the possibility of leak-through.

My mother needs diapers and we found that the kind that you tape on (like they used on her at the hospital) did not stay on her when she got up and walked around. So we use Depends. I know, they suck, but there aren't a lot of options for adult incontinence. We also buy the washable underpads (you can get them for a really good price on Amazon.com - lots cheaper than the medical supply place) and use those on her chair and on her bed. They keep the bed dry (for the most part). At night we double-diaper her. She always soaks through the inside diaper, but often the second layer protects the bedding. And, of course, we have the waterproof washable bed pad too.

A big THANK YOU to the people who posted on this site about Abena Abri form X Plus briefs!!! I had never heard of them, but the comments on this site and also on Amazon prompted us to try them. Mom prefers them over Depends. She says they are softer and so much more comfortable than Depends. So far we haven't been able to test a big poop attack, but for urine they keep her dry overnight and keep her bed dry too. YAY!!!! They're more expensive, but if they cut down on our laundry and other cleaning they will be worth it! Best price is on Amazon.

I'm going to look into these Abena briefs myself for my mom. If they're soft and comfortable, and better than depends, plus keep her drier, that's a win.