To answer your first question about the priors, the answer is definitely yes they can use them. Your second question regarding the canine brutality needs to be addressed with a civil rights lawyer. They can tell you if he has a case regarding brutality.
Yes, they can use his out of state convictions against him.
As for being bit by the canine, you don't say it in your question so I assume your b/f was either hiding or running from the police. You used the words "pursue". I don't believe there is a case for police brutality under these circumstances. Lesson is don't run from the police next time and canine won't be sent in to apprehend.
What you do is hire a criminal defense attorney ASAP. You both don't talk about this to anyone except his attorney. Yes, the State can and will use any and all prior convictions, from other states and from when he was a juvenile, against him for sentencing.
If he was running from police and they sent a dog after him, he's probably out of luck, but he'd want to talk to an attorney who specializes in that area of the law to be sure. But police are likely immune from that sort of lawsuit- sorry to be giving you bad news.
You need to be represented by an experienced and competent lawyer. The questions you ask are too complex to answer in any real detail and it sounds as though he needs representation now.
Can prior convictions be used? Sometimes. It depends on the purpose. It's fair to say that at trial they cannot be used except to admit that he has prior felony convictions unless another reason is offered for their use. It is also fair to say that they can be used at sentencing if it gets that far.
But before that, he needs to have someone investigate the case and defend his interests. Drug dog cases have many avenues for defense litigation that need to be explored. As for the injury, suffice to say that how the injury eventually plays into any cause of action against the city or the sheriff depends in large part on the outcome of the criminal case. In other words, the better the criminal case turns out, the better your chances of being compensated for the injury. So, get the best lawyer you can for the criminal case.
Speak to a few criminal defense attorneys, don't discuss the details, facts or opinions on the case with your fiance if he is in the jail, nor should he discuss the case with others or the police. Good luck.
The above is provided for educational purposes only and is not legal advice nor makes you a client of the Mosca Law Firm, PA. Please consult with a lawyer in order to obtain confidential legal advice that is tailored to your specific situation and facts.