my 28 mnths old son wakes up every 1/2 an hour during night

Dear doctors, please I need help with my 2nd DS.
He is 28 mnths old and he recently got the habit of waking up, about every 1/2 an hour during his night sleep. The pattern is like this: I put him into his bed (he has his own room) and then he goes asleep on his own without crying or yelling at about 9 pm.
Yesterday he woke up at 9:45 then,10:30, 12:00, 12:45, 1:45, 4:00, 4:30,  5:00 and finally 9:15 in the morning. All the times he woke up, he called mommy come here, where are you? without any screaminig at first and if I didn't show up after some secs he would begin to cry and scream. I went to his room all the times and he told me that he wanted me to cuddle him, he drank some water which always is in his crib, and then I would leave him awake said "go back to sleep .-yes mommy" and he would sleep again on his own till next hour...
This has been going on for the last week, although we haven't changed anything in his night routine or in his room and of course we all explain to him that mommy and daddy and brother are all sleeping right now and please don't yell in the middle of the night and of course the answer is always "yes mommy I won't call for you again , i will now go back to sleep" till next episode...
My husband and I are so sad b/c he's not our first and we also had night problems with our elder son's night sleep but nothing like that... We don' t know what else to do. I also thought of getting him sleep with me in my bed so I can get some sleep myself too...
Do you have any answers on that? Might there be anything serious going on with my baby?
Please answer. I'm looking forward to your answer and thanks in advance.
Elina