Is the day sleep still on track?
Anything else going on? Any changes to feeding or temperature or visitors/activities? Teething?
Have you tried a W2S at around 30 mins after BT? Go in quietly and either begin shush/pat or be ready (hand hovering over him) for the transition to come and sooth him through before he fully wakes. You might need to continue for 20 mins until he is in deep sleep again.
If you try this for a few nights and it is not successful then I would probably extend the last A time of the day by 15 mins and see if that helps. I'm aware your LO was doing some long A times already, if BT ends up too late it might be time to cap the last nap of the day a little. If you are still running it as a full nap of 1.5 to 2hrs this might be a time to try reducing back to a CN. If it's a CN already it might be time to extend the other A times in the day and move to 2 naps.
let me know what you think, we can look at it together