Does he seem in pain? Have you tried to offer any type of pain relief at any stage? Did it change anything?

If he seems in pain I would think teeth or an ear infection would be the things to look at first. What is happening on the teeth front? My DD1 acted very much like you describe for about 2 months before she got her first tooth - it was horrible Fine during the day but waking up screaming several times a night and some times screaming for hours. I wasn't really sure if it was teeth or not but I put some teething gel on and she calmed down soon after so I realised I was on the right track. Some nights all we could do was hold her and try and distract her because that seemed to help a bit. I remember one night in desperation I grabbed an empty envelope with a clear window and some how that distracted her enough to stop her screaming. That damn envelope was all we had for many many night time hours, by the time the tooth arrived it was a tattered shread of paper with a scrap of plastic attached but it was the most important item in our house.

Other than that I would say separation anxiety but that sounds a bit unlikely if he is the same when you are with him.

Hope it passes soon.....x