It sounds like you have worked SO hard! Good for you! SO exciting.
You said you don't really know what more to do, and the first thing that came to my mind was, "Stop weighing him." I don't know if that is bad advice - someone can come shoot it down if it is! But I have a slow gainer who was also big at birth and is now almost 5 months old, and I finally realized that I was driving myself too crazy worrying about weight gain, so I stopped doing weekly weight checks about 6 weeks ago and have only done one since then. And I don't plan to find out her weight again until her 6-month check-up. I can see that she's meeting milestones, growing out of clothes, getting a bit chubby under the chin, etc., and she's happy. She was also a constant nurser until very recently. So to be honest, my advice would be to stop thinking about it and try to relax and enjoy the nursing time! But I hope you get some other replies, too.
Good for you for working so hard for your son!