Saying “The not tall boy” instead of “The short boy” does it have a name?
If you’re using what I understand to be the ‘classic’ rhetorical terms, I think this is litotes: using a negative statement (‘not tall’) to convey an understated positive one (‘short’). This term is included on this Wikipedia page with many others: https://en.wikipedia.org/wiki/Glossary_of_rhetorical_terms
(As an aside, I want to note the structure of your example is a little unusual to me. I think I am more used to seeing something like, “Is he short? Well, the boy is not tall.”, as opposed to “Where is the not tall boy?” )