It is fine to give him formula on the days you are at work. You should be able to give him a bf in the morning, and then after you get home, so it would only be one, or two feed each day that you work. It shouldn't disrupt your supply, as you are only working a couple of days per week.
Instead of trying to give him a bottle, try him on the Heinze "firsts" cup. They have a smaller one from 4 months of age, but my son has been on the larger one (from 9 months & holds 280mls) since he was 6 months old and drinks out of it fine. They are non spill, so there is no worry about them tipping it over and spilling. They are priced around $8 for the smaller one, and $13 for the larger on (Qld prices) and are sold at Coles and Woolworths supermarkets. I highly recomend them. Good luck.
Mother of 3, Qld