My 15 year old is a sophomore running on the Varsity Cross Country Team. Last year he did Very good running with freshmen. So well that this year he is the #3 Runner on the varsity team. He is not doing so well because he is running against bigger stronger opposition this year. He is tall and lanky, I have been encourage him to start doing some type of training and diet to have some muscle gain. I feel he is getting bullied at the starting line because he is easily pushed or boxed in when the race starts. If he had more muscle i feel he wouldnt be pushed inside so easy and I feel he would be able to finish races stronger. He says muscle and gaining weight will slow him down. What is your opinion
To answer your question directly, I'll point you to a previous question about incorporating running and strength training together.
More broadly though, I think it's worth noting that the cross country coach at your local high school probably isn't a high level trainer putting people into the Olympics (or even an xc scholarship).
If you want to make the biggest dent, I would find a coach for the non-xc season who can develop your son more directly and work with him for the ~9 months when most high school xc runners slack off or at best are just doing unstructured running. There are some online coaching services that you can consider, or possibly you can find a high quality team and talk to one of their coaches directly.
As a former junior athlete, I can definitely say that the defining characteristic for high level juniors is whether or not they're adhering to a professional program. Hodge-podging stuff from Runner's World and a part time high school coach will definitely make him competitive, but you can do a lot more.
When you look for coaches, look for ones who have proven experience with your son's age group and your son's distance(s).
Running in a crowd is not necessarily dependent on having large muscle mass, but rather technique and being willing to stand your ground. Brent is right, the race isn't won in the first 800m, but it can be lost. I would assume that the coaches are doing drills and workouts to emphasize being able to near sprint out of the gate for position (400 - 800m work) and then be able to settle into a race pace. Cross country starts are kind of like the quote from "Days of Thunder" - "No, no, he didn't slam you, he didn't bump you, he didn't nudge you... he rubbed you. And rubbin, son, is racin"
I would echo the statement that if he is doing that well as a younger member of the team, he has talent, and the best thing to do is let the coaches do what they are paid to do. There are many of us that are/have been runners (I ran x-country through college), but we don't know the specifics of the team training program. We could recommend things, but we don't know if that would help or hinder.
It's also possible that growth is having an impact, and things will settle a bit when that slows down. Just relax, and if it keeps going, just (gently) let the coach know you have concerns about his starting techinque. Most likely they have it down as something to work on already. :)
We have had two high school Cross Country runners with one a senior this year.
First: Let the coach plan the weight workouts. They will know best what the individual athlete needs and I would hate to have a public forum dictate what a high school athlete should be doing while not running. If anything they could do basic core workouts without weights. This would include planks, situps, pushups etc. I would look at what the collegiate athletes are doing and what they look like. I think you will notice they are all pencil thin.
To answer the second part of your question: General a Cross Country race is not won at the start. I am assuming they are running a 5k and they have a mass start. Your son will have time to move to the front of the pack with in the for 800m. If he is anything like runners in Minnesota, to win he will have to be around 16 minutes and he will drop many of the other runners by KM 3. (They will be running a little faster than a 5 minute pace.) Our first meet the winner of the race won with a pace of 4:55