Burglary of a conveyance, while not as serious as burglary of a dwelling, is still a felony charge and depending upon his criminal punishment code scoresheet score could subject him to prison time if he's convicted of that charge. You mentioned a sex charge from 2007, but you didn't say whether he was ever convicted of that charge. Please keep in mind that even if he had adjudication withheld for such a charge, the charge will still be scored on his scoresheet and that will affect any score for his burglary case that he's presently facing. Whether or not the State would be inclined to offer him PTI, I cannot say as that is within their discretion, but if it is offered in this case he would be well advised to take that offer. In any case, if he does not yet have a lawyer, he should immediately consult with an experienced criminal defense attorney to discuss his options.
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Burglary of a Conveyance is a 3rd degree felony punishable by up to a maximum of 5 years prison, while the petit theft charge is a misdemeanor depending of if it is a 1st degree petit theft or a 2nd degree petit theft (which depends on the value), but the maximum will be 1 year county jail for the petit theft. You say that this is his first offense so I am unclear what happened in the sex charge from 2007. As far as a PTI program is concerned that is up to the discretion of the state attorney's office. You need to speak with an experienced criminal defense attorney as soon as possible because you must remember that evidence that can help your fiancé could be lost within 30 days of the arrest.
PTI is probably not going to be available due to the prior. However, it can't hurt to try for it. Of course the charge is serious, but the much bigger problem for him is that once he is convicted of a felony sex charge, and if he picks up a new felony, then he faces civil commitment which could potentially mean a lifetime detention. Get an attorney immediately.
To answer your question we really need to know what his prior criminal record is. The burglary of an unoccupied conveyance generally will not result in incarceration in Hernando County unless there are prior issues. Is he currently on probation for the sex offense? If he is than he should expect a violation of probation also. If the case was dismissed or dropped than it will not be a factor. PTI would really depend upon his record and the circumstances of his charge however, generally speaking it that would be an unlikely result. You should consult with an attorney who is familiar with both the judges and prosecutors in the county he is charged in.