Your situation sounds exactly like mine when we transitioned our daughter. We had bought the convertable crib thinking it was the best bet for growing years. Then when it came down to it, it just looked so far off the floor even at the lowest setting. We looked at the rails etc. etc. but here's what we ended up doing.
We took the side rail off (she was thrilled and even asked for the end rails off too). She had a ball climbing in and out of it and realizing she now had "freedom".
When bedtime rolled around, we put two pillows on the floor next to the bed. For about a month she would crawl out of bed, get under her bed and use the pillows on the floor to sleep on. There was a dust ruffle on her bed so the only thing we could figure was that she felt safe being surrounded by that dust ruffle. We'd let her fall asleep then move her into her bed. She only fell out of bed 2-3 times total. And we left the pillows down there for probably 6 weeks. She quickly learned about the edge of the bed and now has no problems.
Now, on to the second part of your question......how to get them to stay in their beds? I laugh hysterically here.....because at our house that's a joke. The answer is....you don't! We've tried everything from staying in there till she falls asleep to waiting for her just outside her door to threats to treats and nothing seems to work except for sheer exhaustion. Some nights she's ready for bed, other nights she gets up 10+ times before she settles down.
Once she's in bed tho she doesn't get up and roam like I've heard other parents say. So yours might or might not.
It's a big step for your son(and you). And he's probably going to love it. Good luck!
D.