Laura, I am assuming he's been transitioned to a big bed?!?
Do you have a gate? Is that something you would be comfortable with? With the gate, my dd understood that she doesn't HAVE to stay in her bed but she needs to stay in her room at bedtime. Many many nights she spent falling asleep by the gate!!! And for the 1st month (no exaggeration) her bedtime was delayed from 1.5-2hrs. She didn't sleep during the day and it was very tough on both of us (her not sleeping enough at night and being cranky all day and me dealing with her crankiness all day!) but its something that did pass & it will for you too.
Some things that made the transition easier for me was to get the gate (as mentioned above). When she was distressed by the gate (crying, etc...), I would go to her, tell her it was night time, stay with her until she was calm and then leave. With every subsequent cry/call out, i would just repeat the process.
I also introduced a picture chart of our bedtime routine (draw a bath, draw a book, draw a cup for milk, and finally draw a bed) and with every stage we would tick it off... so I guess it prepared her more for bedtime.
During the day we talked alot about staying in bed and how mummy needs her rest and she needs her rest... I would role play with her and ask her to show me how she falls asleep (close her eyes), how to cover herslef with her blanket, etc...
I made sure she had all that she could possibly need at bedtime (water, potty, her lovey, her books).. to minimise the requests we had at bedtime (stalling tactics).
With every return to bed make it as boring as possible.. so no talking or anything like that (other than your good night phase) and honestly, its something that he will get bored of. It is VERY normal for toddlers to keep coming out... especially if its a new thing. He's now got the freedom of coming out... so he's making the most of it.
Is he napping at the moment? The timing of the nap is also important as if you are putting him to bed too early (not enough A time), he is not going to be tired enough to sleep... hence the delayed bedtime.
There is a link at the top of the page as well in regards to successful sleep trainig stories (big bed transitions). Have a read to see if something else there helps you out
hth a little
Layla