Our DS is also 3.5 and also wakes during the night so you are definitely not alone there. Unlike your situation tho, he has always been a night waker and we have had everything from night terrors to screaming for attention to stealthily creeping into our bed at night. We have also tried everything to get him from stop waking including mattresses on the floor in our bedroom, us sleeping next to him on the floor when he was in a cot/bed, controlled crying , blankets on/off, lights on, music on, chiropractor,tiring him out with excercise/activities even a sleep school lady at our house. Despite all of that he still wakes during the night!

It has taken us a long time to get where we are today - acceptance. We dont like that he wakes during the night but we have accepted that he does and we now simply deal with it. Lucky for us for the past year he doesnt wake screaming or crying, he simply comes into our room and he will then either climb into our bed (which is subsequently very squashy as DH is 6ft5 and DS is a very long boy) and sleep with us or one of us will go back with him and sleep the night with him in his bed. Its the only way that he will sleep. We think it is a comfort thing for him as he really likes to cuddle and kiss as well.

I think that during the middle of the night we found that DS was too tired to listen to reasoning - actions seemed to speak louder then words. I would be inclined to perhaps try a mattress by the side of your bed for a while and see how that works. Start him off in his bed so that he knows that is where he is meant to sleep and then if he carries on let him sleep next to you. Not ideal I know but when you are desperate for sleep sometimes you will try anything.

Anyway good luck and remember that no you are not alone!!