FWIW, at 12 months I was giving 3 BFs in 24 hours, and never had a supply problem. At 18 months this went to 2 BFs in 24 hours, and at 22 months only 1 BF in 24 hours, and there was still milk there. I think that past about 10-11 months, for many women, milk is just created on a supply and demand basis (ie however infrequently you nurse, the supply adjust to it). I know that's not the case for everyone, but just letting you know that it is not a foregone conclusion that dropping BFs will decrease supply.
If you wanted to start getting his tummy accustomed to cows milk, you could also try giving it in his breakfast cereal.