[X] do a general tone up with his body like improving his circulatory system fixing weaknesses in his bones cleansing his organs of any contamination and clean his lungs things like that. -[X] Take your time it's much harder to fix any mistakes that you make on him.
[X] Geckos have super wall climbing abilities. You want that, so not only can you hang around on the ceiling, but you can give hugs that people couldn't everget out of.
[X] See if you can expand your senses. Humans can only see three colors, how lame is that? And then there are all their other senses that are even weaker, leaving them effectively blind to the world compared to various animals and bugs. And don't get you started on the senses that people just straight up lack, like a platapus's electroreception.
[X] See if you can get a Darwin Bark Spider. It's silk is REALLY tough, and you could use it to weave a subdermal mesh which will, hopefully,make you effectively bullet and knife proof.
[X] Stalk Investigate Bakugo. You want those stem cells, there has to be a way to get them. If all else fails, maybe you could blackmail ask him nicely.
[X] Maybe you should learn how to fight. There will probably be bad guys you can't just bite, so it'd be good to learn to punch them in the face. De-Kun probably could use practice getting used to his new body too.
[X] Get some Bullet Ants, and learn how they make their venom. Unlike the venom of your friends, Bullet Ant Venom is non-lethal allowing you to use it as a hero. Ah, you can already hear the bad guys screaming in pain.
[X] Improve his circulatory system and immune system allowing it to scrub his body of toxins much faster. Like Lactic Acid. Don't need heroes getting tired after running a couple dozen miles.
[X] With his permission, remove his need to sleep. It'll be a big lifestyle change, but will grant him eight more hours in the day to do stuff, and will keep him from getting tired when he really needs to do something.
[X] His bones can use some toughening up, he can't fix them like you fixed your tail every time it broke. Luckily, you can do something very similar to the disease that gives weak bones, to give him strong ones!
[X] Dramatically increase his growth and other related hormones,and make his production of said hormones seemly and nearly,but definitely NOT endless. Because it time this shrimpy shrub to become a mighty redwood.