Only an attorney fully familiar with his situation and history plus knowledge if the local system could begin to hazard a guess. Help him by making sure he has a good attorney to help him. Good luck.
Mr. Jones has it right. Your Boyfriends attorney will be in the best position to advise him what the probation officer is recommending and what to expect.
It can depend upon the Judge and your boyfriend's prior history. I assume he has a fairly large history, because 2 years probation for simple possession of marijuana is not common for someone with a small record. I anticipate that there indications there may be a drug problem. Addressing this may help before a potential violation hearing.
This is not intended as individual legal advice and there is no attorney client relationship established by this answer. It is advisable that you seek individualized legal assistance. This is not a substitute for hiring an attorney.