((Hugs))
How long have you got until you return to work and when does your DS have BFs at the moment? Most people find it easier to switch one feed at a time so their supply can adjust gradually and there's less chance of engorgement etc. wait a few days/a week and then switch another.
Does your DS take a bottle already? If not, you might prefer to offer the formula straight from a sippy cup.
Also, lots of mamas keep the morning and/or evening bf when they return to work so if your schedule allows this and you'd like to, there's no reason why you'd have to switch all BFs to formula.
If you need some help with getting him to sleep without bfing then I'd recommend you start a separate post on the general sleep board here:
General Sleep IssuesHTH