How to ask your Boyfriend to prom

My school Prom is coming up and since my boyfriend dropped out of school before he even started high school he never got to experience prom. It's something he's wondered what it would be like to go to with me (and I know he's wanted to go to but doesn't want to look unmanly by talking about it). So how should I ask him in a really romantic, sweet and non expensive way because I'm poor?