I saw the movie called “Son of Rambow. Why the spell”Rambow” instead of “Rambo”?
I just looked this movie up on imdb.com and apparently it is a comedy so it could be parodying/spoofing Rambo and several other movies. Not much explanation on imdb but this is what I took from your question and the movie description. It could also be for copyright reasons and wanted to stand out on their own yet draw people with the title.