I'm so sorry.
But you did everything you could. You took him to the vet! You couldn't have known. And if it happened that fast, maybe the disease was worst than expected. You gave Simba a family and all the love he could have asked for.
Believe me, if Simba could do it, he would thank you for all you did for him. You were a great parent to him. And I'm sure he'll be patiently waiting to see you again someday.