I read many of the posts in response to your question. I have only one consideration to add - and no solutions to offer, other than to buy him his own cheap chair to use.
--- Please note that this is not meant to discourage anyone from adopting an older dog! ---
An older dog may lose control of his/her bladder and you may not know it for a while. Our dear departed dalmation had lost bladder control for a few weeks before we actually knew it. We found out because, although not 'allowed' on the couch, she used it at night when we were sleeping. One morning I saw her slink off of the couch as I emerged from my bedroom.
I gave her the usual 'what were you thinking, getting on the couch' yell, the one she looked repentant for when caught but ignored completely after we went to bed. Then I went over to the couch to brush her hair off of it and felt warm wetness. That's when we learned she had an incontinence problem.
The vet said there was nothing he could do for her to correct it, so we had her put to sleep. It was really hard, but it was also the right thing to do.
Afterward, we tossed the couch. A few weeks later, we removed our 'well aged' wall to wall carpet to re-sand the hard wood floors and found that there were 'reminder stains' in all her favorite sleeping spots. In our case, it was time to get a new couch and re-do the floors anyway, so it was not really an extra expense. For others, that might not be the case.