Is this night terrors then? You hear him grumbling a bit, and rolling around, then he's suddenly properly howling, and when you go in he's sitting up, distraught and in floods of tears.
Pick him up, cuddle him and reassure him for a minute or two and then he wants to go back in the cot. Eyes are shut before his head hits the pillow. Sometimes he's doing this a few times a night. Oh, and DP won't do - it has to be me or he gets even more upset, even though he goes down for naps with DP no problems.