What Could Cause My Child To Become Stiff, Foam At The Mouth, And Have Rapid Eye Movement During Sleep, Then Wake Up Weak?

For the first time in my 10 year olds life, I tried to wake my son up from a nights sleep in the morning...He would not get up. His older brother reported to me he was foaming at the mouth and his eyes were closed but moving... I took a look in the bedroom and He was off the bed. I tried to get him up to place him on the bed but he was very stiff...he was foaming at the mouth and his eyes were closed tight but moving rapidly... Once he officially woke up he was very weak...said he was dreaming of drowning and he could not breathe... Took a nap and it happened again only this time he woke up himself... He is in middle school and has to get up early...and he tends to go to bed late at about 10pm sometimes so I know he does not get enough rest... What could this be other than a bad dream...