I don't think anyone is suggesting buying Duplo for this child, only that he may still play with Duplos he already has and may not be ready for "older" Lego sets.
I agree that all the guests may be thinking, "Ooh, Lego theme-- I'll get him some Legos!" and that something completely different might be best. If you really want to do Legos, the most generic general brick set you can find might be best. All Lego fans can always use extra pieces that don't necessarily go with a theme.
I remember a birthday party for DS's friend--the boy got 7 of the exact same Star Wars Lego set. (Yep, it had been on sale that week.) Thank goodness we had enclosed a gift receipt. Then again, it was an X-wing fighter, so maybe he just kept the "fleet"!
DS '04 "Boogaboo"
DD '08 "Lilybear"