So some possible reasons why Morty Jr. is in the hospital:
1) He's dying of old age. This is probably the most likely considering how fast his species ages. :(
2) He needs medical treatment. Maybe like an organ translpant or blood transfussion? I feel like him having half-human half-gazorpian dna would make it hard for him to get medical treatment. (Maybe this also becomes a way for Morty to show Morty Jr. more about Gazorpazorp; considering Morty Jr. probably is not very in touch with his gazorpean heritage)
3) Morty Jr. has his own family now and he wants Morty to meet his newborn child.
THE THIRD ONE WOULD BE SO CUTE
I WANT IT TO BE TRUE
PLEASE LET IT BE TRUE
THE REALITY WAS SO MUCH BETTER, I LAUGHED SO MUCH
Was anyone else hoping the post credits scene would show at least a few of Morty Jr.'s children (since apparently there's a LOT of them)?