If 1) you are on probation, and 2) you violate that probation, and 3) the prosecution is able to prove that you violated your probation, then the judge is allowed to put any legal sanctions on you that are allowed under the laws in your jurisdiction. You'll need a lawyer, but unless the lawyer can prevent the prosecution from proving that you violated probation, the lawyer's job is basically to ask the judge to go easy on you. And that's not much of a defense in most jurisdictions.
Possible sanctions include: Jail time, a lengthened period of supervision, additional community service hours, increased reporting, mandated drug and alcohol treatment, etc.
As an aside, if you are not smart enough or disciplined enough to complete probation, don't go on probation! Tell the judge up front and take your punishment (probably jail time) early, and get it over with.
A local attorney in your juridiction should be able to tell you the likely results of your situation, but in the final analysis, it's up to the judge.
As far as what the boyfriend can do, the first thing is for him to decide whether or not he wants to take jail time, or beg the judge to give him another chance and keep him on probation for a longer period of time, with increased supervision and restrictions. But if the judge gives him another chance and he screws up again, it's not going to be pretty.
This could very well affect his DP. It could result in a finding of guilt against him on the DUI and the sentence for the DUI being imposed in addition to any outcome related to the possession charge. Possession of less than 40 grams of marijuana in WA is a misdemeanor, punishable by a minimum of 1 day in jail and a fine of not less than $250 (maximum 90 days and/or $1000 fine). He should consult a lawyer. I and others would be happy to discuss it with him further. He can try to fight the possession charge or other options which a lawyer would discuss with him after reviewing his circumstances.
This answer is not legal advice and is for general informational and/or educational purposes only. No attorney-client relationship is formed between you and Sean B. Malcolm, an attorney licensed in Washington. For more information, see the Avvo.com terms and conditions of use, which are incorporated by reference herein.
A deferred prosecution in WA requires that the defendant complete a 2 year treatment program and not commit any criminal offenses for 5 years. The good news is that there is not a mandatory revocation because of this new charge. However, your boyfriend will have to disclose this to his treatment agency and likely be bumped back up in frequency for treatment and also have to deal with the new criminal charge. There will be a hearing in front of the judge and it is still possible that his deferred will be revoked. He needs an attorney. If his DP is revoked he will be sentenced to the DUI as charged. Depending on how many prior convictions he has, he could be looking at significant jail time.