I'm reallys orry, but if ds had fallen from this sort of height and was dopey, and still listless and not himself the next day I'd either get an emergency GP appointment or take him A&E.
If MIL is not great with these things then to be honets I would wprry about leaving her in charge of babay as she needs to be able to cope with emergencies...
Sorry not be offering the reassurance you were after. I lean towards the more laid back attitude and am not given to over-reacting/panicking, but if you have a "niggling doubt" then do something about it...