Sounds like it could be Gas, or constipation
Try giving him more water during the day, especially before bedtime. and less milk,
Milk can constipate, you'll notice his poo will be light and sort of milk chocolatey ish LOL
or black and pebbley,
Try bicycleing his legs or massaging his belly,
or pushing his knees into his stomache
to help move the gas, or BM
Some children have night terrors,
My kids have had this on occasion, what worked for me was to turn on the lights, and TV, till they fully wake up, and then turn off the lights, and watch Tv and rock them til they relaxed, then I would bring them cuddled in my arms to my bed.
I would also turn on and off the lights walking around the room, expllaining all the shadows,turning the lights ON and OF on and off to show and then i made shadow puppets with my hands, and last I buy a Dream catcher for the window.and nightlights. scented because its relaxing and comforting.
Good luck
M