Hi, sorry can't be of help with experience, as only had a one dog household. But am thinking of 2 in the future. Did a little search and found some articles. Hopefully board members who have dogs, especially introducing older dogs can give you their experience and some advise.
Here's what I found:
Introducing a New Dog to Your Resident Dog | petMD
Introducing Your New Dog to Your Other Dogs : The Humane Society of the United States
Wishing you luck and fingers crossed you can help this ole guy enjoy his remaining days with you and yours.
KAZ