Malamber
What about pokemon and digimon transformations? And if the knowledge from another world's species works can't any imagined species work? What about ben 10 aliens, marvel mutants, inhumans?
He can transform into an alien species with super intelligence and create his own super serum or compound v and increase his capabilities beyond limits. There are also different species with superman abilities like metroman from megamind and megamind species is also very intelligent.
Malamber:Off hand i would say Mutants and Inhumans would be a no go because at their base that are still human and his human form is Gar. Ben 10 aliens would probably be redundant because of all the alien life in this universe. Like I have said before Imagination will be a major key in the later transformations but the MC hasnt watched every bit of Anime or cartoon out there. Sorry to say but his imagination will be limited to mine with help of my readers.