I had a similar experience with DD. She would feed for ages and extremely frequently. I could never see her swallowing, and my boobs rarely felt full or leaked. Her weight gain was very slow for the first 4-5 weeks (she never lost weight after the initial loss in the first few days but did drop 2 or 3 centiles).
I had a fab HV who kept a close eye on her due to the poor weight gain, but reassured me she wasn’t really concerned as she looked healthy and was having plenty of wet and dirty nappies.
We knew that her constant feeding and poor weight gain were probably due to her bad tongue tie, and things improved once that was snipped at 3 weeks. Has your DS been assessed for tongue tie and has your latch been checked? I’d recommended a breastfeeding support group / lactation consultant as tongue tie can often be missed by midwives.
If he’s throwing up a lot he must be swallowing some in the first place. Don’t take not leaking or feeling full as a sign of no milk - DD was EBF til weaning at 6 months (still feeding at night now at 1) and I stopped using breast pads completely at about 3 months.
Also, they will guzzle formula. It’s far less effort for them to get that out of a bottle than it is to get BM from you, and it will always satisfy them longer than BM as it slower to digest. Taking a lot of formula isn’t a sign that you don’t have enough milk.
I found the first 6 weeks or so horrendous - it was relentles, I found it very painful and had cracked nipples, and I was so anxious she wasn’t getting enough as I couldn’t see the evidence. It absolutely does get easier though! DD is now the most efficient feeder ever and climbed back up the centiles well before starting solids.
I remember the worry well, hope things get easier for you soon.