Have you tried almost no wind down? He sounds like a spirited munchkin (lucky you with two of them, eh?) and may do better without the reminder that he's about to go to sleep - I know DS certainly did.
Rubbish weather here too, torrential rain or scorching heat...
Not a lot you can do about him picking up on your stress, hopefully some of it will be gone soon... have you tried writing it all down before bed or in the morning before anyone else wakes? I found that helpful, esp. when I burned it afterwards, or stabbed it with a pencil til it was just tiny bits of paper!
How about deep breathing exercises - in through the nose, out the nose - may even be calming for Levi if you walk around with him while doing that.
Guessing you don't get a lot of time to just sit and relax yourself into a trance-like state - I lie down nice and comfy, eyes closed and think about my favourite place to be, count backwards from 10 thinking of different aspects of said favourite place and find myself relaxing almost subconsciously. Even if you do it to help you relax into sleep at the end of the day, it may help. I find I can visualise thoughts running across my sight, but they're just colour blocks and somehow my mind clears a bit.
He may not need low stimulation all the time - maybe just before naps... Would Caleb sit and read with Levi, or some other relatively quiet activity - singing songs, animal names/sounds matching with pictures, tell him stories? I know its a bit much to ask him to be still all the time, but what about that as the last activity before nap time, so he's still around the family and not doing a 'proper' wind down, but still getting one IYSWIM.
