A parent on FLR and a child on tourist visa

[Pepe n Pilar]

if he is over 18 he is not a child to the uk gov

his mom has flr and married to a brit ?

Thanks Mr. Joebloggs for the reply. Yes he is not a child (minor age). So he will apply on a tourist visa following all the requirements.
His mum has the FLR visa ang married to a Brit.
What additional requirements should he present to the Embassy? Who is best to make the sponsorsphip letter for Him?

Thanks.