Sounds like you're doing so well, mate. They are all over the place in those first six or so weeks. I too think that the MCHN is just looking for problems for you, even if she was concerned I think it's too early to think of all the feeding issues you may come across in future! Things are going great now and that's what matters.
I also would love him to be able to go a bit longer between feeds, as I am struggling with having an 18m old and a constant feeding newborn... Things are not fun at my home at times...
It is hard for your DS1 right now. But DS2 will probably settle a little more over the coming few weeks. I found that DD1 needed a lot more loving from other family members when DD2 was just new. Then, once Phoebe was having longer sleeps I was able to spend heaps more time with Natalie again. I find it ebbs and flows, sometimes one needs me more than the other. Phoebe is pretty needy right now and probably things will be different again in a few weeks... It's just the way it is when you're part of a family. What I absolutely LOVE is that they are great mates. Natalie loves her sister to bits even though it seems Phoebe is what takes my attention away from her. And Phoebe, well, no one else gets the smiles, giggles, belly laughter and excited-to-see-you squeals that Natalie gets.
So it is tough at the moment, but things will get better and you will get better at managing the two of them.