One of the things that has always angered me a bit as a band director is when I see students (or adults) being thrust into playing an instrument that is not right for them simply because "we have Uncle Ed's old trumpet in the attic." In reality the selection of an instrument for a beginning player should be based not only on what the student wants to play, but also on the physical characteristics of student himself.
For example, in general students with full lips will have great difficulty playing efficiently on a trumpet due to the very small size of the mouthpiece. Likewise, a student with very thin lips will not likely do well playing a large bore instrument such as the tuba. Other issues such as dental work (or future dental work such as braces) or physical conditions such as asthma can all limit the extent to which a student will be able to succeed.
Unfortuntely, more often than not I see parents forcing their child onto an instrument that their band directors have warned against. A few years later, maybe even less, the student will drop out of band out of frustration. Don't let this happen to your student. Listen to your local band director and trust in his or her judgement. You both have the same goal in mind, to see your student succeed. Making the correct choice from the very beginning is the best way to make that happen.