I adore the name [name_u]Freddie[/name_u]. The problem is, I don’t like the name [name_m]Fred[/name_m]. I’m worried that if I put “[name_m]Frederick[/name_m]” on my son’s birth certificate, people will call him [name_m]Fred[/name_m]. So I’m tempted to just put “[name_u]Freddie[/name_u].” But I also want him to have the option of [name_m]Frederick[/name_m] when he gets older, if he wants a stronger, more grown-up sounding name. I’m so torn! What do you think? [name_u]Freddie[/name_u] [name_u]Mason[/name_u] or [name_m]Frederick[/name_m] [name_u]Mason[/name_u]?
Use [name_m]Frederick[/name_m] since you like both [name_m]Frederick[/name_m] and [name_u]Freddie[/name_u]. I wouldn’t worry about [name_m]Fred[/name_m], it’s very easy to correct people. My name, for example, is very easily shortened to [name_f]Beth[/name_f] and I can’t stand [name_f]Beth[/name_f] so I correct people when they call me that. The only concern you should have is the fact that he could choose to go by [name_m]Fred[/name_m] rather than [name_u]Freddie[/name_u] or [name_m]Frederick[/name_m] and there’s nothing you can do about that if it’s what he prefers. [name_m]Frederick[/name_m] [name_u]Mason[/name_u] is very handsome 
I also say [name_m]Frederick[/name_m]! Best to give him the choice. And [name_m]Frederick[/name_m] [name_u]Mason[/name_u] is lovely
[name_m]Frederick[/name_m]!
[name_m]Frederick[/name_m] is my vote. Whether his birth certificate says “[name_u]Freddie[/name_u]” or “[name_m]Frederick[/name_m]” there’s going to be the possibility of people calling him [name_m]Fred[/name_m]; what matters is what YOU call him. I think if you solely call him [name_u]Freddie[/name_u], people will do the same.
[name_m]Frederick[/name_m]! Not a big fan of [name_u]Freddie[/name_u].
[name_m]Frederick[/name_m]!
There is a chance you call him [name_u]Freddie[/name_u] and people will still call him [name_m]Fred[/name_m]. You just have to make your preferences clear.
I personally think [name_u]Freddie[/name_u] is too nicknamey. [name_m]Frederick[/name_m] is gorgeous, classic, more formal and more suitable for an adult, while keeping the chance of calling a little boy, [name_u]Freddie[/name_u].
PS. And [name_m]Frederick[/name_m] [name_u]Mason[/name_u] flows better than [name_u]Freddie[/name_u] [name_u]Mason[/name_u].
I would go with [name_m]Frederick[/name_m], and then just enforce the use of [name_u]Freddie[/name_u] only - not [name_m]Fred[/name_m] - as his NN. I don’t think it is very fair for a person not to have a full name as their legal name. It makes it hard to sound professional as an adult.
[name_m]Frederick[/name_m].
I think [name_m]Frederick[/name_m] to provide a formal option.
You run the risk of the nn [name_m]Fred[/name_m] with either name but if you always introduce him and call him [name_u]Freddie[/name_u] the vast majority of people will follow suit.
I am a [name_f]Joanne[/name_f] who really dislikes being called [name_u]Jo[/name_u] (too tomboyish for my personality). I always introduce myself as [name_u]Josie[/name_u] or [name_f]Joanne[/name_f] (in more formal situations) and so all of my friends refer to me as that without question.