Are you traveling in a 6-person compartment with bench seats? If so, those bench seats usually slide out and down to make a more or less continuous bed at seat level. If there's no one else in the compartment, you could let your son sleep there, perhaps stuffing any remaining gaps with pillows or coats or whatever. I wouldn't count on too much sleep, though.

Good luck,

Michael

p.s. perhaps something like this?