My 10yo ds is having his 12yo friend over tonight for a sleepover, but we don't really have a good set up. Both of my boys share a small bedroom, and each has a twin bed. I could put our extra twin bunkbed mattress on the floor between the two, but it would be a bit tight.
We have a rec room downstairs, and I could put our camping sleeping pads down there and have the boys sleep in sleeping bags. The rooms not really all that put together - just a bunch of toys every where.
When my ds sleeps at his friend's house, they share his double/queen bed, which we don't have.
How old is your other DS? Young enough to sleep with you? If so, I'd have the sleepover in the bedroom, and the friend can take your other DS's bed. Alternatively, ask your 10 year old what he'd prefer to do. My son is 9 and would probably go for basement camping.
Let the friend sleep in his sleeping bag on the floor next to your son. Or your son can give his friend the bed. or they can both sleep in sleeping bags in the living room (not the rec room). Kids don't need a mattress the way adults do. Have fun!
or - what about a tent in the back yard?
Most sleepovers for me growing up were in a sleeping bag on the floor (except the odd friend whose family had a sofabed).
I'd pull out the camping pads and let them decide which room they'd prefer. Tents make for a fun sleepover too (as pp mentioned).
I would ask them where they would like to sleep. If they are like my kids, they'll opt for the basement camp out.
If they decide they would like to stay in the bedroom, I'd offer the guest the bed, and your son take the floor.
If your "other" son (not sure if he is older or younger than the 10 y.o.) doesn't mind, he could take the sofa for the night. I'd only do this if he really wanted to move out of his room for the night. It would be with the understanding that he'd get the same courtesy when he has a sleepover friend.
DD only has a twin bed in her room. When she has a sleepover, they often put out sleeping bags in the family room. It's easier to watch tv. into the wee hours of the night that way (she's a teen).