Formula in a sippy cup


I'm a first time mum with a 6 month old who is still fully breastfed. I have tried multiple times to get him to take a bottle but I've had no success. I've tried different bottles and teets and also tried expressed milk and different formulas. Is it ok and has anyone else given their babies milk from a sippy cup rather than bottle? I'm trying to find something that works as he currently feeds every 2 hours still which means i can't leave him with hubby or family if he needs feeding. (he loves holding and sucking on the sippy cup already)