Oh Sarah... that must be so hard... *hugs*
Off the top of my head, the only thing I can suggest is can you have soft calming music/lullabies playing in his room on repeat, so when he wakes he will hear it and maybe start listening to it and drift back to sleep again before he calls to you? (Have no idea if that would work in reality, but just thought I might suggest it)
I haven't had this precise problem before (ours usually involve Aidyn coming into our bed), so I'm not sure what to suggest, but I will have a think today and get back to you if I can think of anything else...
Hang in there hon....