First of all was he weighed on the same scales each time? This can make a big difference and good practice is alwats to use the same scales. Also if HV put the scales on carpet or uneven floor it'll be inaccurate
secondly how are the nappies? Lots of poo and wee means he's taking enough milk in.
How is he ih himself? Happy? Alert? All good signs.
Does the latch seem good? No pain?
No dehydration is really positive as it means he's getting enough fluids which, unless you are giving him water, will mean he's getting enough milk.
Stick with it, you are doing a fab job and your son is lucky to have a mum who wants to get this sorted for him. Of course you've enough milk, people breastfeed twins so please don't worry!
I had the same worries with slow milk coming in and a big baby, and I remember spending a weekend crying because I thought I couldn't feed her. But I could and 8 months later we're still going and she's 23 pounds!
If, in a week he's still not gaining, please don't give up. Ask for a peadiatricc referral to work out what the issue is. It could be something as simple as a tongue tie, if there IS a problem.
My money's on inaccurate weighing though. My DD was once weighed and it was out by almost a whole pound so it does happen.