I was in this position exactly with my newborn earlier this year. I know how desperate it feels. Hang in there. It's a short phase and it will get better.
We too were put on a feeding regime at 5 days. My milk was slow to come in after a c-section so baby had lost 12.7% and was too slow to gain for their liking so we were made to feed every 3 hours, then top up with whatever I'd expressed, then top up with formula. It's gruelling and didn't help my milk to establish. The formula will make baby feel full and they then don't make as much effort on the boob to give the milk the message that it's needed (so as long as you are topping up with formula, the expressing is vital to get that message across).
I would give your baby the formula just to please the doctors, make DS put on enough weight that they'll let you get home, hopefully within 48 hours (this is what happened to me). Once you're home I'd remove the formula (maybe gradually rather than suddenly?) and let your milk take over within a week. I didn't do this (because the bloody hospital put me on the feeding plan but didn't tell me how or when to come off it) and it wasn't until 4 weeks that I just dropped the formula altogether and lo and behold my milk suddenly fully established!
Be prepared for your community midwife and health visitors to want to perpetuate the feeding plan as they'll be wary and want to follow the doctors' orders. But this is your baby and if you want to breastfeed you will need to take control of the situation a bit, which I eventually did but I regret waiting 4 weeks to do so.
Make use of your time in hospital by asking for as much help with breastfeeding as is available. There were a couple of excellent feeding experts at my hospital who helped a lot to give me the confidence that baby was actually getting something from me.
Drinking plenty of water and eating well will help your milk supply.