I agree with previous posters. You probably should have some plan. First, it will be easier on your boobs to produce at certain times of a day, rather than any time randomly. Second, you may end up soon giving him more and more formula as you won't remember when you were supposed to nurse and when to offer bottle.
Breastfeeding is a demand and supply thing. The more you supplement the lower your supply eventually will be. You can keep a balance if you are determined to do it. The best bet would be to have some plan, and stick to it. If you skip nursing and offer a bottle, try to skip next bottle feeding and nurse instead. I think, we have here quite a few moms who were able to breastfeed and offer a bottle of formula in the evening and did great. Just be prepared for growth spurts, if you supplement during them, your supply won't catch up with your baby's new demand.