Hugs, its going to take more than just 1 day to see improvement. You're trying to change the co-sleeping association and it can take anywhere from 2-4 weeks to see improvement (it took us about 2 weeks to sleep train and mine were much younger than your LO).
If he's only taking a short nap in the morning, offer the afternoon nap around 3.5hrs later and if he wakes 45mins into the nap, try to resettle him for another 15-20mins and if he doesn't return to sleep, I would probably do 30min earlier bedtime.