I have a few tips that might be helpful!
I saw another person on reddit (u/TygerWaffles) who has some birds with physical ailments, and something they did was use a small detached door from a cage for the bird to perch on so they felt more comfortable and stable than if they were just on a perch or finger if youre moving them around or keeping them with you. You could also create a bird stand/play gym with a bottom platform thats maybe part of a cage or something. An option would be to buy a small, lower-end cage off craigslist or facebook marketplace that can be easily disassembled and use the pieces for something like that.
I have a budgie who cant fly so I bought a cage off Amazon thats much wider than it is tall so that if he falls it isnt very far, and putting a small blanket or dish/bathroom towels on the bottom of the cage helps for if they fall too.
Amazon and bird supply stores sell platform perches like this one https://www.amazon.com/Borangs-Platform-Playground-Accessories-Anminals/dp/B078NPRDWP/ref=mp_s_a_1_4?dchild=1&keywords=platform+perch&qid=1612759651&sr=8-4 (although there are several different styles and sizes), and they can be a great place for your bird to rest while in the cage. Id maybe get a few for him and hang some toys by them where he can reach so he has a few little spots to sit and play if he wants.
But im not super knowledgable about/experienced with birds with splayed legs so Im not sure if these are the best recommendations. I would check google too if you havent!
Note: I was just reading the description of this subreddit and this sun deck is basically the small platform I mentioned that the other reddit user utilized for their bird!