An electric cello may not weigh much...but the cord to wall power, or the battery, and the speaker, probably do. :/
A full size violin, back in the day, might only weigh a pound...but the case easily weighed 3. (Played as a child). Given a full size cello is several times the size of a violin, I'd say at least 5 lbs ( over 2kg) for the instrument at a minimum.
And, being real, for every person in an albergue that likes cello music, there is as likely to be one who hates it.
The OP said that her son wanted to carry it, and play it every night. IMHO, if he's up for a mixed reception, and understands that he'll do it without pay, *and* won't make his parent carry the other gear or do his noghtly chores?
As long as they're not planning the Ingles in early June, no skin off my nose.