If you went with Morgan I think people could possibly question which one was the boy (assuming they knew you had 2 girls and 1 boy), so if you think that would bother you then Morgan wouldn't be a good idea. I do agree with southern.maple though and think it would work really well with Delaney and Ryley :) - and I would also assume Ryley spelled that way was a girl, but maybe that's just me.
What about Marvin?
I was going to suggest Marcus until everyone else suggested it and you said you didn't like it much, but I thought I'd add that I know someone with a son Marcus and they call him Mac (part of the reason I thought of it for you). I love this name and it seems to sort of fall into the same category with Gabriel and Noah, so maybe it wouldn't be that tough of a sell for your hubby. I also seem to recall coming across the name Macarius, which kind of sounds like a Bible name (and might be -- I'm not up on my Bible studies), a la Gabriel and Noah, and it gets you the Mac nickname if you want it.