Depends on which son you're referring to, but basically you'll need to get rid of gavelkind succession as soon as possible. If you can't reach high crown authority during the life of your current ruler, choose ultimogeniture (youngest child inherits) if you have multiple kingly titles, or elective monarchy if you only hold one kingdom.
On a side note, the way gavelkind succession works seems completely messed up to me. There's absolutely no logic as it comes to which titles are given to the other child(ren). It'd be great if you could at least choose which ones will end up with your heir.