Hi. Welcome. I am a Licensed Master Social Worker with more than 20 years experience working with individuals and families on a variety of issues.
What your son is experiencing is quite common at this age as their dream life becomes active and they can experience nightmares
in vivid detail. You said it perfectly when you said he doesn't want to be held during this time and that is because he is most likely still sleeping
. He most likely doesn't even remember these events in the morning. What you can do in those moments which feel very scary is go into his room and gently whisper that everything is okay and give a gently rub his back for just a few seconds. I would let him continue to sleep.
Was there any trauma prior to him being adopted? How old was he when you brought him home to you? If you believe that he could be re-experiencing some earlier trauma then I might suggest touching base with the pediatrician about it.
Just to reassure again that at this age these kind of nightmares are common at this age as fears develop, imagination comes into play and they can describe their dreams.
Here are two articles on nightmares and night terrors for you to look at to see which describes the event. http://www.babycenter.com/0_nightmares-why-they-happen-and-what-to-do-about-them_64922.bc
And one on night terrors. http://www.parenting-ed.org/handouts/night%20terrors.pdf
Please let me know your thoughts but again most likely this is a normal part of his development.