Unfortunately, it may require the dog viciously attacking someone or some other animal to get anything done about it.
It appears Fair Oaks is serviced by Sacramento County Animal Control.
The dog is committing this offense of the Sacramento County Code:
8.08.056 Dogs at Large.
No person shall permit or suffer a dog to stray from private property owned or legally possessed by the dog owner or the person who has a right to control the dog unless the dog is restrained by a leash or lead not exceeding eight feet in length, except in the following situations:
1. When the dog is assisting a peace officer who is engaged in law enforcement duties or when the dog is participating in a search and rescue effort at the specific request of a law enforcement authority;
2. When the dog is enrolled in and actually participating in a dog training or obedience course, exhibition, or competition conducted by an organization on private or public property with the permission of the owner or operator of the grounds or facilities;
3. When the dog is assisting the owner or person in charge of livestock in the herding or control of such livestock; or
4. When the dog is accompanying and under the direction of a person engaged in hunting on land which is within a restricted shooting district as defined in Section 9.40.060 of the Sacramento County Code. (SCC 0815 § 2, 1990; SCC 576 § 9, 1983; SCC 544 § 3, 1983.)
Which means it is a "Public Nuisance":
8.08.110 Violation a Public Nuisance.
The maintenance of any animal in violation of any provision of this Title or any order of a Hearing Officer issued pursuant to this Title constitutes a public nuisance. (SCC 0815 § 2, 1990.)
In addition ...
8.08.050 Prohibited Conduct.
No owner of any animal, wild or domestic, shall permit or suffer the animal to do any of the following:
1. Be at large (excepting the domestic cat);
When you write your letter, you might point out to your neighbor that even if he does not technically hold title to the dog, he is considered the "owner" for purposes of the Sac Co. Code:
“Owner” means the primary or responsible person who: (1) possesses, has title to, an interest in, harbors, controls or has custody of an animal; and (2) feeds or otherwise provides care or sustenance to the animal for thirty (30) consecutive days or more on property located within the unincorporated territory of the County. (SCC 1356 § 7, 2007: SCC 0958 § 3, 1994; SCC 0815 § 2, 1990.)
You may need to educate your Animal Control Officer that he/she can write a citation to your neighbor.
Be sure you make it known that you are willing to testify against your neighbor.
I would encourage you to continue to complain, as much as you can. Take pictures, and write down when these incident occur. If you aren't getting cooperation from Animal Control (who are usually very lazy and largely incompetent, based upon my personal experience), you may need to raise a stink with the head of Code Enforcement for the County, or perhaps his/her boss, or the County Commissioners. Be sure that you have documented the incidents, and your calls to Animal Control due to the dog being at large.
It appears the Sac. Co. Code has very little teeth (pardon). There just isn't much there to help Animal Control deal with this problem. Evidently, the County Commissioners do not believe pit bulls (or other vicious dogs) are that much of a problem.