SICA

Hi all, apologies if this appears in the wrong forum, i'm a bit lost with all of this. I hope that somebody may be able to give me some advice...... I am looking to apply for a passport for my 6 month old baby boy, but i'm a bit lost - d'uh! I know how to apply for the passport but is there anything else that i need to do to register him as a Brit? Do i even need to register him as a Brit? His older brother (yes, my other son) is 5 and a half and has both British and Thai passports but i can't for the life of me remember how i did this! Is it just as simple as applying for this passport for my youngest? EDIT - i should have said that I and his mother have been married 6 years - if that makes a difference?! If anybody has any recent experience or knows this system and would like to share i'd be extremely grateful. Many thanks in advance