Because you have not indicated that there is any litigation involving him, Jeff cannot subpoena this information. If Jeff is charged with a crime or is a party to a lawsuit, that person may be able to issue a subpoena for those records.
Because the Freedom of Information Act applies only to federal agencies, it does nothing to tell us what records are or are not available to Jeff.
Jeff's most straightforward path to this information is instead through a Public Records Act request under Mont. Code Ann. §2-6-1001 and Article II, § 9 of the Montana Constitution.
Depending on the source of the sheriff's information, records about Jeff may be presumptively available or presumptively unavailable. In most cases, Jeff's rights to them are going to be analyzed using a balancing test that weighs his right to know what the government is doing against other interests, such as the public's interest in the integrity of criminal investigations or the privacy rights of other people identified in those records.
So if Jeff sent the sheriff an e-mail confessing to cheating on a first-grade math test 40 years ago, there's probably not much to outweigh his interests in access to that information. But if an undercover officer has infiltrated Jeff's sleeper terrorist cell, Jeff probably won't be able to get copies of the mole's notes about him.
There are also some states the provide the public with the right to access information about themselves held in government databases. If Montana has such a statute, that could potentially also provide Jeff with the right to access some information about him.