I made diaper pads for my paraplegic doggie I used the tops of panty hose and velcro I would put part of a diaper in the front and it worked pretty good Now they have Depends which you might figure out how to use for him, The tops of panty hose might keep the Depends stable I tried several things before i figured out what was best for my doggie,Go0d luck, let us know what you do.
There are also doggie diapers advertized but i dont know anything about them
Bless you for your patience and love for your old doggie