I have had my Russian tortoise for about two weeks.  All he has wanted to eat is the spring mix lettuce.  He seems to have diarrhea.......Any suggestions on what to do and what to feed him ?

I would say this has been caused by the lettuce, I would pick weeds and feed him tortoise friendly plants and flowers, there is info on this site including pictures.  If he's not keen on the weeds mix with some lettuce but reduce the lettuce over time. 

Wendy