a song of ice and fire - Why is Jorah Mormont called Ser when he’s from the North?

Jorah Mormont is from the North (from Bear Island) and they are supposed to worship the old gods.

Being knighted requires a Septon and the Northeners don't believe in Septons, I guess. I thought knighthood was mostly in the rest of Westeros.

Answer

Knighthood doesn't always require to follow the Faith of the Seven, all that is required is another knight to hear you swear your vows to the Seven.

Knighthood is intimately tied to the Faith of the Seven, and the ideal of knighthood is to be not only an honorable warrior but a devout follower of the Seven.

Eddard Stark himself was not a knight, because he proudly continued to worship the Old Gods of his ancestors, yet still a great warrior. Sometimes, however, a soldier who does not follow the Faith of the Seven may nonetheless be rewarded for exemplary service with the title of knighthood by a lord who does worship the Seven. Examples are Ser Rodrik Cassel and Ser Jorah Mormont, both of the North.

^{_{— Knighthood, Game of Thrones Wiki (source)}}

Note that though the wiki seems to imply that only soldiers can be knighted, any man can either buy knighthood or earn it with a great deed (as Davos Seaworthy did, who was a smuggler, not a soldier).

Jorah Mormont received a knighthood from King Robert I Baratheon, in recognition of his bravery during the siege of Pyke.

According to Illyrio Mopatis, Jorah was anointed by the High Septon when he became a knight.

Illyrio whispered to them. "Those three are Drogo’s bloodriders, there", he said. "By the pillar is Khal Moro, with his son Rhogoro. The man with the green beard is brother to the Archon of Tyrosh, and the man behind him is Ser Jorah Mormont".
The last name caught Daenerys. "A knight?".
"No less". Illyrio smiled through his beard. "Anointed with the seven oils by the High Septon himself".
^{_{— George R. R. Martin, A Game of Thrones, Chapter 3 (Daenerys), Series: A Song of Ice and Fire - 1}}

It's customary to be anointed by a septon, but not always required either. The only requirement seems to be another knight to hear your vows, and witnesses are generally customary to add validity to the knighthood.

“Hmpf.” The man Plummer rubbed his nose. “Any knight can make a knight, it is true, though it is more customary to stand a vigil and be anointed by a septon before taking your vows. Were there any witnesses to your dubbing?”
^{_{- The Hedge Knight}}

_{^{Credit: see more detailed info on the different ways one can become a knight, and the intricacies in this answer here, and how the procedure has changed over time in this answer here, two great answers provided by @Aegon.}}