I know this is frustrating. But it is a good sign that he is covering up when you put the feces back in the box. I would get TWO BIG boxes and put one up and one down. give him plenty of opportunity to defecate in a box without much work or decision making.
Defecating outside of the litter box has many causes. Luckily it is not as serious a problem as urinating out of the box, because cat feces are easy to pick up and clean up after.
Causes are many; changes in litter type or style or smell, medical issues (like impacted anal glands), environmental changes (noise, competition, fear), age related "forgetfulness", and so forth.
Could even be that after his declaw he was extra sensitive and that caused it.
First try to figure out what caused the problem. Then do your best to correct that. Obviously those corrections are many - depending on the problem.
Veterinarians can examine and if necessary express anal glands. After that, there may be a week or two of residual mild pain or funny feeling in her perianal area but over time hopefully this will go away and he will return to normal full defecation in the box.
If the problem is caused by something that has created a new behavior pattern then we simply try to "re-train" her. We use the same process no matter what the cause.
Pick up where she has gone, place it in the box for about a day. Clean the spots well (usually does not need much) and use some enzyme stain and odor remover. (Nature's Miracle or Simple Solution are the products I've had most success with) Use by label directions.
If the spots are near the box, you can make those spots unappealing by using aluminum foil on the surface, or some other material that is not attractive for defecation. (you'll have to experiment.. some use plastic garbage bags, astro turf, etc.) Some find that physically blocking the area with furniture or another object will literally block that area and force them to go back to the box.
Scolding or any negative reinforcement of this problem usually makes it worse, so don't do that.
This is usually a problem that can be corrected in a young to middle aged cat. Try these suggestions and I hope it helps.
Actually, this could be a litter problem. In my experience, it's not uncommon for cats to have different preferences regarding both location and texture for urination and defecation. Our cat used to urinate in one box and defecate in another pretty consistently.
When you say your cat is now defecating on the floor, does that mean tile? carpet? or something else? His past behavior of defecating in the bathtub indicates he seems to prefer smooth slick surfaces for defecation and if the "floor" he is now using is also smooth/slcik that would be consistent.
If so, try adding a second box near the current one (and/or next to where he's defecating if these are 2 different locations) that has very little litter in it. We generally recommend no more than about an 1 1/2 of litter in a box but I'd use less than that for his "defecation box" - maybe just enough to cover the bottom of the box. If you modified his litter at all after his surgery that may have also contributed to the problem.
Be careful about covering the soiled area with an unpleasant texture if it is right next to the litter box. - you don't want him to accidently step on that when he's trying to get to the box. If there is a soiled location away from the box try an upside down carpet runner - there are sharp plastic points that most cats don't like to walk on.
Finally, do not try to show him the box or put him in it or have any unpleasantness associated with the box. Discipline "after the fact" doesn't work and if he begins to associates you-feces-litterbox with unpleasantness you will have more problems.
There is a reason for this behavior that makes sense to your cat, it's just your job to figure out what that is!! Be patient and keep thinking like a cat