About two years ago, my sister Erika gave our nephew his first set of Legos. At the time, Lego had a line called "Quatro" that was twice as big as the Duplo and aimed at younger kids. My mom was quite disappointed to learn that they have discontinued that line. Then I taught her the wonders of EBay.
Mom bought two lots of Quatro (about 35 blocks total) for Anthony to play with. The first batch of them arrived today. Anthony can easily take them apart and can put the two bump size back together.
The Quatros fit together with the Duplos so he'll be able to continue using them when he gets bigger. (Anthony already has his first set of Duplo blocks, they were a gift from Celisa's kids for his birthday.)