Oh he's adorable. What a delightful photo of you and your beloved "Little" (aka Poot.) Now we know some of his and your story. You can see in that photo just how completely over the moon you were at the time about having him in your life. And it is so evident in your words how much you loved, cherished and adored your boy.
Like your "Little" (aka Poot) my cat was an orange and white tabby that I named "Marmalade." I witnessed him use up at least 4 or 5 lives right in front of me. He was a true survivor, and we had many adventures which I have written about some of here on this forum, but he too was sick in the end and becoming a shadow of his former noble self. So I also had to let him go. It broke my heart into 1 million pieces and I am still struggling to put those pieces back together. But I am doing better than I was. Now I mostly think of our time together with joy and smiles and even laughter at times.
Marmalade was once the King of a colony of feral and stray cats in the high desert country of New Mexico, U.S. He was feared and respected by the much larger and healthier males (all Marmalade had to do was regally raise a paw up slowly, and not even strike any of them, and they would cower) and was loved by the females in the colony who trusted him with the kittens as he was never unkind to the females or the kittens, even those he was not the Father to. Towards the end of his life he had gotten into a major Tom Cat fight with a rival who had attacked his girlfriend cat "Star" (our neighbors cat from next door) and Marmalade was wounded in his left ear which never healed correctly which required surgery. This lead to his health deteriorating. I owed him a peaceful passing.
I am so sorry your boy fell so ill and that you had to do what needed to be done. But I'm so grateful that your paths crossed when they did and that for so, so many years you two had each other. All cats and humans should be so fortunate.
The depth of your grief is the depth of the love that Poot felt from you for most of the days of his life. How wonderful must that have been for him, to know how much he meant to you. You gave him a family, a friend, love, affection, safety and purpose.
Kind regards & my sincerest condolences,