Hi Kayz
We started doing the same thing at about the same age as your LO. DH gave one bottle of ebm or formula once a day for the same reasons as you, to get him used to the bottle and to give me a break. If I was to do it again I would of not given formula as it became way too easy to give. And the weaning process started earlier than wanted to as the milk supply started to reduce, and DS started refusing me. Then he started being fed formula more and more. My DS is 6 1/2 months and will only take 1 bfeed in the morning and is fussing for that one. I think we will be completely done soon. As for formula we tried everything imaginable, as DS has reflux. The easiest on his stomach was Alimentum by similac. It's a hypoallergenic formula. A bit expensive, but worth it. I'm not sure if you're in Canada or the US, but if you are from either place you can get it. If not any other hypoallergenic brand is easiest on baby's stomach.
I didn't mean to ramble but I wanted to let you know that sometimes when the bottle is introdused more often it sometimes results to weaning before you are ready. I know that weaning wasn't your question but I thought I'd share our experience since we started the same way as you.