I don't advise laying with him and/or going back and laying with him if/when he gets up. It becomes a "game" and then HE is in control (I want mommy; I'll get up and she'll come back.)
I agree with the crib tent/canopy idea.
When you do get to the point of putting him in the toddler bed (we gave ours their "big-boy bed" for 2nd birthday), you can simply "lay down the law" that it is bedtime and he is to stay in the bed. After the first few nights of getting a "surprise on the bottom" each time he got out, my son got the idea. Sometimes just a firm "NO" and putting him back and leaving the room was all it took.
The best advice I can give is to establish a ROUTINE that makes his little body KNOW it is bedtime (example: bath, pj's, brush teeth, kiss daddy, read story, mommy leaves, sleep). Until you get it down or even to reinforce it to your son, a picture symbol chart is great. I have used these for my son with autism (mentally 2 to 4 yr. old); I adhere velcro to the back of each picture card and move it to a different column until each card is done.
Funny story: My older son was really good at staying in the bed. One night after I left the room, he fussed and fussed. It was not like him (he was about 2 1/2). I finally decided something was wrong and investigated. I asked him what he needed and he pointed to a stuffed animal that had fallen out of the bed. (I had a hard time stifling my snickers.) It was just out of reach, but he was NOT going to get out of bed to go get it. As soon as I left the room, I burst out laughing. :o)
It sounds like you are doing a GREAT job if your son is going right to sleep. :o)
P. <><