My two were on formula for what seemed forever! I don't think they weaned until they were 10-11 weeks old. The mammal babies book says you can wean them at 8 weeks, but my two were still on 4 feedings a day at that age and when I tried to cut them back, they lost weight. Do you have a gram scale to weigh your little tyke on? I would weigh him everyday and try cutting his milk back slowly (like give him maybe .5 ccs less for the first day and see how he does over a couple days then give 1 cc less and see how he does and keep going until he is weaned) and see how he does.