Erg workout to lower my rowing 2k?

I am a 17 year old boy, who rows competitively. I made it to scholastic nationals in my jv 4+, and we got 19th out of 36. My 2k is only 7:08, and I am having trouble geting it below 7. I am looking for advice on a good weekly workout to get my 2k below 7 minutes and hopefully closer to my goal of below 6:40 (I know its a longshot) by the end of the summer. I am 6 ft 3 in, but I am only 160 pounds. I am incredibly skinny and lanky, but I don't want to row lightweight. I have been told by doctors that I need to put on weight because it is unhealthy. This has been a problem because I have to compete with people that are 180-220 pounds, so I have been trying to bulk up over the past few weeks. Thanks for the erging advice, and I hope to accomplish my goal.