He's not a bad looking horse....
Built butt high and a sacroiliac joint further back than optimum makes him appear wonky and very weak in his hind-end.
A few pounds needed and some muscle tone blended together this horse would be stunning.
What concerns me is you wanting to give a retirement home and still ride him in the same thought.
Depending upon how much riding, how strenuous, how often, what kind and how large a rider you are this older gentlemen may be a perfect match or a don't touch.
Retirement normally means slowing down and can't keep the same amount or type of riding he had done.
So, if a jumping show horse he was, becoming a trail riding partner may be perfect for him.
For me, he looks a kind soul of good size and bone.
He actually doesn't appear terribly thin, but lacking a bit of weight and muscle tone especially along his top-line makes him appear poor...it is looks though and can be changed.
As horses age they also sometimes face a harder time keeping weight on, need special diet considerations and better care honestly.
I have older, "retired" horses in my herd.
They are the best trail horses who will walk for hours, trot or gallop occasionally if the group does but mentally are unflappable and know how to take care of inexperienced riders...just nice horses to be around.
Depending upon what "retired" means to you....
For me it means not only do they need, require more care, they will be laid to rest when appropriate and not be sent to a horrors unknown at a sale.
If you are willing, able to take on a older horse, accept them for what they can give, and give them a peaceful existence when all they can give in the end is a nicker of hello...then take him on.
If you are looking for the "retired" horse to go showing, campaigning on and hard rides his own body is no longer capable of, not be able to meet his older bodies food needs nor his extended care needs or more often vet care...then pass on him and look to buy a younger horse who can be ridden daily and do all the things this horse has limitations on because he is aged...
Remember that depending upon his age you could have many years of togetherness and riding if you recognize limitations and work with them not against them.
I expect my guys probably have 8 - 10 years left of riding and life with me...and I will do all in my power to make those years kind ones for them and when time comes they will leave this world with a hug, kiss and tears in my eyes cause they are just special souls and I know it.
It is all up to you in what your expectations are of the animal and what he can do for you, as a team...
It is also all on you to recognize the limitations he may have now and accept only what he can give today and what may change tomorrow...
There are no guarantees in life.
Today the horse may be ride-able, tomorrow morning something occur and never ridden again, for any animal
That is when you figure out just what it means to be "a caregiver" and what is best for the animal at that moment in time.
Some serious deep thoughts to what you want, need and are willing to accept you face before taking on this animal or any animal.
If you think you want to be his human, get a vet out to help determine if he can meet your needs as a partner and can you meet his needs as a owner. Called a PPE or health assessment.