You should start by showing him where in scripture it teaches of the assumption of Mary so that you can definitively show that Catholics can prove all of their beliefs by scripture.
http://www.scborromeo.org/papers/assume.pdf
is about as demonstrative as the books of the Bible say. However, the Church has beliefs in parallel with Scripture, for example, the Didache, that did not make it into final Scripture, that is still held to be true.