I feed my horses (14.2 and 14.2hh very light exercise in winter) half a square bale a day in the coldest months when they have zero grass (literally - because it's under several feet of snow). However, given that this is your first year of having your horse at home, and since he is bigger than mine, I'd play it safe and buy about 3/4 to a full bale a day. Depends on the size of your bales, but it sounds like it will be the same hay you're using now, so you have a pretty good idea of how big they are. Incidentally, when I boarded one of my horses, my BO told me they were feeding him 3/4 of a bale a day. No way that was true. He gets fat if he gets a little more than half a bale.
But consider that there will be some waste. I also use slow feeder haynets, and wouldn't do it any other way, but occasionally, I will find a bale that has more weeds, or is coarser, and I set it aside for other uses. Some still ends up on the ground while filling the haynets. You may also need more hay than you thought in colder weather. There may be environmental factors that require you keeping your horse on hay longer than expected (a spring drought, a hay shortage next spring). Worse comes to worse, you end the winter with a bit of extra hay. Better than running out! My first year keeping my horses at home, I bought over 100 bales too much - so I just sold the extra in the spring for exactly the same price I'd paid for it (probably could have gotten a lot more judging by the interest!). I always keep a bit extra too, in case a horse needs to be stalled when they would normally be on pasture. I may be a little over-cautious, but it drives me crazy to see all the desperate people looking for hay in the spring because they didn't get enough, and are waiting for first cut.
Best to have a little extra if you ask me!