Defamation (Libel and Slander. See J. Jonah Jamerson for the difference between the two) is not a protected act of speech because it is speech used to harm someone on claims of actual damage. Since you are doing this in small claims, you simply have to file the suit in your local court to have the case go through (assuming you don't have a lawyer, who does this for you).
The Burden of Proof will depend on Jurisdiction. In the United States, speech is Protected Free Speech until proven otherwise, so you will need to prove this is a defamatory statement. You can get his evidence as to why he's making a truthful statement in the discovery process, but be willing to provide dates and times to show you were not there when he claims you were OR to show that you had a valid non-him related reason to be near him. Depending on how he is phrasing it, if you have not been convicted of stalking and he has not filed any motions against you in court, then you can use that to prove that this is false. If he has, he has the burden of proof to prove you are a stalker (although he doesn't have to say you are an alleged stalker... the media covering the case will say as such until proven guilty.. yes, even if the crime is caught on camera and it's definitely you, media writing rules still call the accused as such until proven guilty).
In the U.K. and other Common Law Jurisdictions, the burden of proof is on him to prove the claim is valid.
In addition, you need to prove that these charges have damaged you in some capacity. A loss of friendship, romantic encounters, housing or employment opportunities or membership to private club or service from businesses because of these untrue statements. At some part, you must show that his statements factored into a negative decision against you (probably to a significant degree).
Also be sure that these are statements of fact (OP is a stalker) and not belief/opinion (I believe OP is stalking me/ I think OP is stalking me) or hearsay ( The First Poster told me that OP is stalking me). The last two statements are technically true in that he has assessed the situation from his perspective and has come to this conclusion OR he is repeating a statement made to him by another source. Only the first statement is false.
Finally, in the United States, you need to prove malice on the part of the guy making the false claim IF you are a public or limited public person (a celebrity or a politician in the latter case, or someone who was temporarily a known person due to other matters like being related to another crime or news story that has put you into the press).
It is Defimation if:
- It is false.
- It is stated as true fact and not opinion or hearsay.
- It has caused significant damage to you.
In the U.S. the burden of proof is on you (U.S. Only).
If you are a public entity (Politician, Celebrity, or subject of press coverage) you must prove intended malice towards you (U.S. Only).