Do another pwc to get the ammonia down to 0 if he's looking poorly.
His system is probably not fully recovered and he needs basically perfect water right now. So his body can focus on healing and not dealing with water parameters which isn't ideal.
Ammonia should be at 0 especially for a fish recovering from illness.
It is hard starting out and not having the best information about fish keeping. How long ago did he have ammonia poisoning?
What size tank is this?
How long has it been set up?
You mentioned Plecos, what type and how many,
and what other fish are in the tank?
Have you read the article in my signature link? Super helpful information and a literal fish life saver!
https://www.aquariumadvice.com/guide-to-starting-a-freshwater-aquarium/
I would probably do a 30% pwc - if you have done a pwc in the past week. Then another one the same 30% in 8-12 hours, and see how he's feeling. Test for ammonia after that pwc. Make sure it is 0.
If you have not done a pwc in more than about a week, I would do a 10-15%, then do another 10-15% right afterwards, called back to back water change. Test and see if there is ammonia. If there is ammonia, then wait an hour or 2 and do another pwc of like 15-20%. Test and should be 0.
Then about 8 hours later do a 20-30%.
Reason is incrementally changing water to gently make a change in the water parameters. Longer than a week and the water difference in your tank and the water coming from your tap may be vastly different. Since your little guy isn't feeling well trying to make his water safe quickly but gently.
Then watch your water parameters especially if you haven't built up a BB colony in your filter for the nitrification cycle to be working optimally. Info on that in the article to learn more.
If you aren't cycled then you need to proceed with a fish in cycle for your tank in general.
https://www.aquariumadvice.com/i-just-learned-about-cycling-but-i-already-have-fish-what-now/
https://www.aquariumadvice.com/fishin-cycling-step-dark-side/
And if the Betta isn't getting better after a day or 2 and the water changes then he may need additional help.
