He will adapt! Try stretching the morning window with some activities, for us that was the key challenge as first window was only about 3-3.5hrs, 6-6.5 for the second was never an issue. It was painful at first as sometimes we would need to go from 11 till bedtime at 6.30 but adapted after a week or so and first window is easily 4-4.5 hours now, second one can go up to 7.5hrs easily 😀
I'm in this too. I feel like this nap transition will actually never end 😭
When we first moved to 1 nap we were in the same boat , wake somewhere between 5 and 6, short morning nap and skipping afternoon nap. We just took the hit and accepted an earlier bedtime for a while , she didn't wake any earlier than she had been and after a few days we were able to start pushing bedtime further back and she started waking later Now we're up to a 7am wake, 12:30pm nap and an 8pm bedtime. Was a gradual process, and a pain at times.