girls may not be ready to get pregnant, but doesn't mean they are not ready to "go" at that age. and unless something is wrong, a 14yo boy will be sneaking magazines and smut channels, and talking dirt talk with the "dudes".
so.... i think your analogy works as to why you should not breed young underweight females. i don't feel this applies to males.
but as others said, it's a coin toss; he could mate, he could freak out, or he could sit there. won't know unless you try, if it's worth it to you to do so.