[name_m]Hi[/name_m]! I love the name [name_u]Liam[/name_u], but should [name_m]William[/name_m] be the full name on the birth certificate, even if he’s always going to be called [name_u]Liam[/name_u]? Please leave your opinion!
I’d say if he’s only going to be called [name_u]Liam[/name_u], it might be easier just to call him [name_u]Liam[/name_u]. Plus, both names are super popular so it wouldn’t be an issue to have [name_u]Liam[/name_u] stand alone as its own name.
Then use [name_u]Liam[/name_u]! It’s perfectly fine as a name in its own right. If that’s what you’re going to call him, then that’s the name you should give him. 
I’m of the mindset that if you’re never gonna call your kid X, make X the middle name or don’t use it at all. [name_u]Liam[/name_u] [name_m]William[/name_m] would be silly, so I’d just use [name_u]Liam[/name_u]
(I have both in my family and they’re all doing just fine with their respective names for what it’s worth)
If you’ll only call him [name_u]Liam[/name_u], go with [name_u]Liam[/name_u]. I personally prefer [name_m]William[/name_m] but voted for [name_u]Liam[/name_u].
I think that [name_u]Liam[/name_u] is enough of an established name that it can stand perfectly by itself without being short for [name_m]William[/name_m]. However, if you’re having a hard time letting go of [name_m]William[/name_m] then I don’t see why you can’t use it on the birth certificate because having siblings [name_m]William[/name_m] and [name_u]Liam[/name_u] is a bit bizarre so you might not get a chance to use it again
Love [name_m]William[/name_m]! But if you really like [name_u]Liam[/name_u] then just use it as a full name.
If you’re going to call him mainly [name_u]Liam[/name_u], I’d go with that
I agree with previous posters to just use [name_u]Liam[/name_u]. It’s very much an established name in its own right. Also, [name_u]Liam[/name_u] doesn’t feel like a super intuitive nickname for [name_m]William[/name_m] to me because I pronounce them differently. [name_u]Liam[/name_u] is like “LEE-um” but [name_m]William[/name_m] ends up sounding more like “will-yum.”
I am an advocate of using the name you want to call him in full, so I vote [name_u]Liam[/name_u]. [name_u]Liam[/name_u] is already the most popular boy name in the US. In no way will it come off as unprofessional.
i honestly it is never my first instinct that [name_u]Liam[/name_u] is derived from [name_m]William[/name_m]. I would always assume [name_m]Will[/name_m] or [name_m]Bill[/name_m] over [name_u]Liam[/name_u]. And [name_u]Liam[/name_u] has established itself like [name_f]Eliza[/name_f] or [name_f]Beth[/name_f]. They can be separate without needing the full [name_f]Elizabeth[/name_f].
I voted [name_u]Liam[/name_u], but I don’t think it’s a big deal either way. If you use just [name_u]Liam[/name_u] he won’t have to deal with the confusion of which name to write down on a form, and he won’t get called [name_m]Will[/name_m]. But people might assume he has a full name of [name_m]William[/name_m] sometimes. If you name him [name_m]William[/name_m] he might by [name_m]Will[/name_m], [name_u]Liam[/name_u], or [name_m]William[/name_m] depending on what suits him when he’s older and can decide for himself.
I do have some experience with this since I’m a [name_f]Katherine[/name_f] nn [name_f]Katie[/name_f] and I go by [name_f]Katie[/name_f] 100% of the time. I sometimes wish my legal name was [name_f]Katie[/name_f] for convenience but I hardly ever think about it. I do dislike having to choose whether to tell people “oh this is what I go by” or just trying to respond to a name I don’t identify with. Mostly this is the case at doctors offices or other places where my legal name has to be on a form. Once I was paged in an airport overhead but didn’t recognize my name for a long time… oops!