The highest tower Nathan built today, before deciding that allowing gravity to exert full control over the blocks used, was 5 blocks. The trick to keeping him building lies in providing him with new blocks before his brain can decide that his hand needs to hit something. Preferably something near the base of that cool tower.
Nathan just managed to stack four Duplo-like blocks on top of each other, without parental intervention. I'm not sure, but should a one-year old be able to do that?
Maybe he was bored because he had to listen me practice my speech for 45 minutes ;-)