But he's already having three meals a day and breastfeeds four times during the day and once (usually) overnight(too much?)
This sounds perfect to me for this age
You're completely right that milk should form the major part of his nutrition until he is one, and in general I wouldn't be cutting out daytime feeds while he still has a night feed. It's slightly tricky if you feel that night feed is a habit/prop rather than a genuine hunger need but personally speaking I'd be aiming to drop that one first when you are both ready before cutting out daytime milk.
In terms of feeding just morning and night - yes your body will completely adjust to that. Lots of people do it
In general when it comes to it you'd aim for dropping one feed at a time just to give your body time to adjust and hopefully avoid too much in the way of discomfort/engorgement.
If you are planning to gradually drop feeds over a few weeks (while he's still little) before your return to work then you will need to replace what he doesn't have in a breast feed with either expressed milk or formula by cup or bottle. Cows milk isn't recommended as a main drink until a year of age. That said, of course there is no magic switch that changes on the day they turn one! There would (in my opinion, others may disagree) be no issue with him continuing on breast milk for now and then starting cows milk a week before his birthday. Or, to be honest, just going with the milk in the morning and at bedtime from a year onwards as nutritionally he can get what he needs in the daytime from other sources. Lots of people feel strongly their LO should still have a milk drink in the day as a toddler, mine didn't and were just fine but it's what you feel comfortable with ultimately
Others may have chosen to do things differently, but I saw 10-12 months as a transition time from 'mainly milk' to 'mainly solids' so we did drop one of the daytime breastfeeds in favour of water/snack when mine were 10-11 months, and then kept 3 feeds until closer to the year when we fairly quickly weaned off the other two (my choice to stop at that age - but obviously no need to do so at all unless you want to!).
Hope that's some help, am sure others will have some (much wiser) words of advice for you soon!