Do you think you can sort of 'force' a nickname?

As above. If the child is introduced from birth with the nickname ("this is Bob" (as opposed to "Rob/Robert") then yes.
Though the only one I actually know has a nickname that's so far from the original that I've always wondered why they bothered with the original and didn't go with the nickname. (Name is Thomas, nickname is 3 letters that nobody would ever associate with Thomas and contains only a T from the original!)

You just tell people his name is Ben. He will think his name is Ben, he will tell people his name is Ben. There is a space on nursery and school forms for ‘known as’.

Just seems a bit pointless having a longer name for the birth certificate in that case.

I think the baby needs to be introduced as ‘nickname’ from day one of you really want people to stick to it and use it. They can have a full or different official name but you don’t need to mention this if you really don’t want people using it all the time.
We did this with my ds. He has a name that we thought might be too young and cute for an adult but used it from day one. He has the option to use his official name if he prefers. (So introduce him as Alfie but official name is Alfred, only issue is new teachers every September call him ‘Alfred’ but that’s not a big deal).

So you want a full name for nobody to call the baby, but just to be a 'formal' name while he goes by a nickname that isn't really associated with the full name?

You could always just put the full name on the birth certificate but introduce the baby as the nickname and not tell them the formal name. Or mention it in passing but stress the nickname will be his used name.

I think you can yes. I know a little Loulou, I don’t know what her given name is, something like Amelia

Absolutely you can. I’ve been known for over 40 years by a nick name which is nothing to do with my name, it’s not a shortening of it, it’s not even a name!