Hi C.
I would recommend the book Personality Plus by Florence Littauer. Your son's personality sounds like my son's - he likes to get his way! After reading this book it gave me a huge insight into how my kids react in the ways they do - a lot of it is based on their personality. By having this knowledge it gave me a better understanding why people react differently to the same situations.
My 9 year old son is also a picky eater but he eats lots of the food he likes, and I always serve him the food he likes. He is offered other foods (which he never takes) and I never force it. He eats: meat, fish, whole wheat pasta, whole wheat/zero sugar bread, cheese, carrots, corn, fat free plain yogurt (mixed 50% with low fat vanilla yogurt), whole oats, rice, cookies, dark chocolate, high fiber/lower sugar snack bars, potato and potato snacks, peanut butter, rye crackers. And everything has to be served separately. I also supplement his diet with organic multivitamins, fruits and vegetable concentrate chewables and Omega 3 DHA - then I know he is not missing out on nutrients. As long as what he is eating on the whole is lower in sugar, low in saturated fats, has fiber and calcium, and he gets adequate water (he hates milk!), I am not concerned, even if there is not much variety in food. I try and make sure that I always have a snack with me for my son, like when I pick him up from after school care, as he gets cross when he is hungry (I am like that too!).
With regards to the negativity before an activity, is it not the same with adults - don't want to go to the gym but when we are there we enjoy it! My son can be a little like that - I ignore it and carry on as normal!
All the best.
M.