He can expect an MTR and that can placed him back in jail for the original offence for the whole time he was supposed to serve to begin with. He needs a law firm that can help him.
Your boyfriend is presumed to be innocent of his DWI. Thus, he is presumed to be innocent of violating his probation. Hopefully, the prosecutor in the county where he was arrested for the DWI does not notify the other county. I am not clear what level of offense he is on probation for at this time. He might want to check with a bail bonds company to see if a warrant for his arrest goes out for the motion to revoke his probation. If that happens he can make a non arrest bond so he does not go back to jail.
Depending why and where he is probation determines what will happen. The most important thing to know is that can fight both cases. Therefore, you should meet with as many criminal attorneys as you can and choose the one you feel most comfortable with after speaking with all of them in person.
First, it is likely a requirement that he report any arrests to his probation officer or it is a violation. I would advise him to look at the conditions of probation.
Second, I would consider whether he has an alcohol problem that could have caused his involvement, if he was involved, in either or both offenses. (Just because someone gets arrested for DWI, even if s/he is guilty, does not mean they have a problem with alcohol. However, if one gets a DWI while on probation, it leads to the conclusion that this issue needs to be considered.)
It is possible that since he had yet to report on his probation, that they may amend his conditions of probation (including what is misnamed jail therapy which really just means to get his attention.)
However, if he is not guilty of the DWI, and he has not violated any other conditions (such as drinking while on probation, failing to report the other arrest, etc.) and the DWI is dismissed or he is found not guilty, then they may do nothing about the original probation.