Since we were told we can do multiple things, here's all the stuff I can think of
[X] Kindly inform his metabolism that it isn't the Stone Age - you can stop hoarding that fat now. Get some muscles instead.
[X] Myostatin in stupid, why should someone have to work for their muscles?
[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] 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] Fix his inefficient stomach and intestines they barely absorbs anything.
Additional Actions
[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] 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] 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] See if there's anyone else with interesting non-quirk physical adaptions.
[X] What would happen if you made yourself a second brain? Could you think doubly good?
[X] Protect your precious cinnamon bun, he can't survive being blown up like you.
[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] 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.
All of the Additional Actions I can think of. Anyone else got any ideas?