How about buying him his own calculator? We gave our ds (now 4.5) one when he was younger and taught him how to key in problems like
1 + 1 = = = = = (keep pressing "=" until your thumb falls off)
to count by ones. It became his favorite car toy (still is) and he's figured out things like skip counting and exponents on his own with it.
I don't know that a calculator really teaches the concept of why 1 + 1 = 2, but it's taught ds to recognize patterns in numbers. Plus, it keeps him occupied on long car rides.
