Thank you all so much for your kind replies and words of encouragement. I'm sorry it's taken me ages to reply, I just thought I'd get a mail alert when there were replies to my post. Thank you for sharing your own stories and I'm so sorry for your losses.
H3mit, I did gather the courage to ask the vet, he said feeding him more appetizing foods would probably not have made a difference, but I'll suppose we'll never know and all we're left with now are conjectures.
Rationally speaking, I know all the "what ifs" in the world can't bring him back now, but this doesn't make it any easier for me to forgive myself.
Hugs to you all, hope you're having a good day!