G.T.
The rule of thumb is to wait till they are a year old. Assuming he doesnt get a tummy ache from it, I suppose you could do it now. I never did the transitional way by adding milk to the formula, I just went to straight milk when that time came without any bad repercussions. It's so nice when you dont have to worry about having formula in the house anymore... now it will be milk, lol. Keep a lot on hand.