I'm with you on this one! Maybe you know this, but we all wake up several times a night. If everything is as it should be, we go back to sleep and never knew we woke up. If something is out of place, we wake up fully. If you are staying with your son when he goes to bed, when he wakes, something is out of place. Your choices are to have him go to sleep by himself or stay with him all night.
My daughter just started staying in her bed all night by herself a few months ago - at 3. For her, it worked to tell her she was going to start school (preschool) soon and if she was not sleeping in her bed, she could not go to school. She stays in her bed about 5 nights a week. Usually if she wakes up to go to the bathroom she crawls in with me and I decide if she goes back in her bed or stays with me. Baths at night never worked for her - they would rev her up. Finding our own routine and staying consistent with that really helped. We brush teeth and wash face, go potty and put on pj's. We lay down and read 2 stories. I then ask her to turn out the light "so her brother knows it's time to go to sleep." I sing, "It's time to turn the light off, the light off, the light off, it's time to turn the light off and go night-night. Daddy's sleeping, and Mommy's sleeping, (insert name here) is sleeping...it's time to turn the light off and go night-night." She drops off pretty quickly most nights. It took a good month to get her used to the routine.
My cousin also went through something similar - her son went to bed fine on his own until they traveled and the baby was scared so they started staying with him to go to sleep. When the trip was over, the new habit stayed.
They would do a regular bedtime routine and put him in bed. They would then say, "I have to go potty. I'll be right back." They would stay out of the room for a few minutes and come back. Then some laundry would have to be folded, etc. They would stay out of the room longer and longer each time. The important part was to actually do what they said they were going to do - when their son came to check, they were actually folding laundry...When he came out, they would say - I'll meet you back in your room in a minute. After a week or so, he would fall out by himself and stay asleep.
From one tired mom to another - good luck!
J.