How about introducing a few one nap days? Do 4hrs 45 again, let him sleep uncapped and resettle anything less than 1hr 30 / 2 sleep cycles and do EBT of 6.30 or 7 depending on day or nap length. I'd aim for a maximum of 5hrs A after a good nap, or a day length of 12hrs max. Then the next day go back to your 2 nap day for a few days to let him recover any OT, then do another 1 nap day when you think he's ready, or if nights start to worsen.