How much is he eating otherwise? (food I mean)
Cow's milk might be ok if it's just a part of a full diet, but not if you plan to replace breastmilk with it (as it is not a complete food for infants). So a bit of milk, plus other food, to replace a feed (if that makes sense?)
Kazzo you may find that she will take it if you're not there and there's nothing else on offer. Or she might not and just wait till you get back.