B.M.
My DD went throught Night Terror stages about 3-4 times...starting at about 2 1/2....she is almost 5 and we haven't had an episode in about 5 months....like the others it starts about 90 min after falling asleep...and lasts about 30 min and then asleep for 45 min and then it starts....we just follow her around...keep her safe....try to hold her if she will let us...occasionally we'd sit on the back porch in the fresh air...of course in the morning she had no recollection....reading about them on the net really helped me realize other kids go through this..my MIL never really understood what I was talking about and always assumed it was nightmares...no not the same...anyway...it will pass and seems to get less frequent as they mature....a friend though has a 18 year old who gets them when she runs a fever...they've had to keep a baby gate at the top of their stairs and even told the college roomates what to watch out for!
Hang in there....make sure you and your DH trade off watching him if you get too tired or stressed...it's so hard watching a little one go through them....interesting though about how it happens on the days your MIL watches him...is he not getting a good nap those days maybe???
B.