Remember to update both your game and graphics drivers to the latest version to check if this problem has been fixed, first.
There could be many reasons for this problem, namely
Overheating: Microstuttering is a very common symptom of overheating. Does the stutter happen as soon as he starts the game or after playing for sometime? Check the CPU and GPU temperatures before starting the game and after the stuttering starts and check how hot the system is running.
Configuration: Another solution you can try is one linked on the official Steam for MAC forums HERE
Edit or create
autoexec.cfg located in
users/your username/library/application support/steam/steamapps/your steam account name/counter-strike source/cstrike/cfg to match the following:
//_Set Netcode vars_
cl_interp "0.033" //try "0.0151515" if that is laggy for you.
rate "25000" // you can try higher, I have mine set to 100000 (probably unnecessary but alwell it is smooth. )