It means they want him to use Legos to figure out math problems. Manipulatives always refer to overpriced Legos bought by the schools from "Durable Hard Goods for Education" companies (i.e. LEGOS).
It has always been my experience that teachers spout educational jargon to drive a superiority wedge into the conversation. When the parent can spout the jargon back (in spades) then the gloves come off and real work gets done.
Also in my experience, asking the teacher if they've read the latest abstracts from the ERIC database on a particular subject gets the point across that I will not be intimidated by their jargon and let's just cut the crap.
(ERIC= Educational Resources Information Center)