I'm thinking of hosting my bf's birthday meal somewhere that doesn't serve pizza because it's all he ever eats.
Why would you not want him to enjoy Pizza on his birthday?
I'm fed up of his unhealthy eating habits. It's like he's a toddler.
Birthdays are meant to be a nice celebration for the person involved. Don't try to force someone to do something they don't want to do on their birthday. They'll be upset at you for ruining their special day.
Friends are supposed to look out for each other.  I'm seeing this as an opportunity to show him how bad his pizza habit is for his health.
There is a time and place for that, but you are picking the wrong day by trying to teach this lesson on his birthday. That is a day where someone should be able to do what they want.