hi. its been a while since i was in menorca so forgive me if this information isnt exact.
if you are going by bus, i would have thought the best way to do this would be to go on a bus from Sa Caleta to Mahon then on a bus from mahon to son bou. this was what we had to do when we went to menorca and wanted to go elswhere, unless the bus goes through Sa Caleta anyway in which case you may be able to just get off.
the Reason i said about mahon then changing is because when we were there that was the main statiom and the place of interchange.
like i say, i havent been for years so i dont know if it is still the same or not, but im sure some one on here will know a bit more than me!!
hope this helps
M