My DP's little girl (not mine) was bought a strawberry shortcake toddler bed by her grandparents, and it's awesome. You use a cot mattress, and it's nice and close to the floor with half rails, so that they can't fall out of bed, and they can climb in and out with ease because it's the perfect height for them.
I'm pretty sure my MIL got it off ebay, but i'm sure their pretty easy to find.
It's not so different from a single bed, but i wouldn't be very comfortable with putting a toddler in a proper single bed as kids roll around (my nose will contest to that after my stepdaughter sneaking in to sleep with us, and me copping a toddler foot to the nose at 4am) and theres a good chance they'll fall out. Especially in the morning when they get up and their still groggy.
Just take it slow with your son. It's a big change for him