Natural Mom, I haven't ECed with a newborn but I have some suggestions
How about just carrying a prefold under him and letting him do his thing on that. That way you would notice it right away. Then, when you see him going, you make the cue noise. That way, even if it's still on a dipe, you are helping him learn the association. Or even if you fasten the dipe, don't put a cover on it so you notice right away and he doesn't sit in it.
Does he scream whenever you take the diaper off? If so he mght be cold? Or is he just not liking the positions you've tried?
Have you tried just holding him, diaperless, in your arms in the cradle hold over the potty (or receptacle)? Is that clear? I've read that's done in some cultures for very young babies because they are so floppy. My ds is 7 m/o and still doesn't care for the under the thigh hold you see so often.
Another thing to consider is, it's NOT now or never
If it just doesn't work *now* try again in a week or a month or whatever. You are still very well within the ideal time (birth to 4 months). And in diaper free I read that even in some traditional ECing societies they use diapers in the very beginning when the baby is so new and floppy.
I don't have much practical advice because we didn't start til 6 m/o but it's going great and I'm happy
So even if it doesn't work now, don't give up forever. Good luck!
P.S., do a search cuz there have been many EC threads lately and some mentioned newborns I'm pretty sure!