When I was 7 and diaper punished I was made to sleep in my old crib.
I fit pretty comfortably and it was actually quite enjoyable.
My mom also had a crib mobile above my bed which also helped me sleep.
I would be put in the crib wearing just a diaper or sometimes diaper and shirt.
A 7 year old should pretty easily fit into a regular size crib.
However you could also buy 2 baby cribs and then join the cribs together.
I did that and made me a larger sized crib.
I am 6 feet tall and could curl up in a regular sized crib although after awhile it could get uncomfortable.
I want to get a larger crib but cannot afford it at the moment.
So I had the idea of buying a second baby crib of the same kind and then joined them together to increase the length of the baby crib.
Now I can more easily and more comfortably sleep in the baby crib.
You could also join 2 baby changing tables together to make a longer one for your 7 year old.
I would also have him suck on pacifiers, feed in baby food, drink from bottles and make him wear just the diaper around the house.
Don't allow him to ask for diaper changes and only you change his diapers for him.
I would also add his name on the nursery wall in block letters and make it a nice baby themed baby boy room.
Place some toy cars, trucks etc in the room for him to play with as well.