A love for salt is acquired, and while he may be resistant, you are his mother, you love him and want what's best for him so keep it up!
With high blood pressure becoming more prevalent even among school-age children it's important to provide your son a lower sodium diet. High blood pressure in turn can lead to heart attacks and strokes later in life, and too much salt can also affect one's kidneys, very dangerous in children because they are smaller Take a look at these articles from various countries, it is a recognized health threat doctors are very concerned about:
http://www.awash.org.au/cons_saltandchildren.html
http://www.usatoday.com/news/health/painter/2008-03-16-sa...
http://www.canadianliving.com/health/nutrition/the_danger...
http://www.dailymail.co.uk/health/article-98813/Have-got-...
http://www.childrenshospital.org/views/june07/pediatric_k... ~ this last link talks about the rise in pediatric kidney stones in children due to too much salt in their diets
So, toss your salt shakers, don't add salt to foods you prepare, and serve fresh foods rather than processed foods like lunch meats, hot dogs, sliced cheeses, and all the foods you mentioned, there is so much "hidden" salt in everyday foods. Continue to serve him lower sodium foods, and make it a family affair, extra salt in anyone's diet is not good. Use herbs, spices and lemon to add flavor to foods.
Salads are a good fresh option for everyone, you can offer fruits, veggies, and protein in them. Also make your son's foods fun and interesting to eat by arranging his food into faces or shapes so he's into eating it and not looking for the salty tastes.
Make your own spaghetti sauce, mac & cheese, etc., so YOU can limit the salt in them. It's not that difficult, I've always done it. I took over cooking for my dad 6 years ago when he developed kidney failure, after having diabetes for over 20 years, and then having a stroke. He developed a love for salt as a child, so I had to totally revamp his diet, eliminate the sodium packed foods, and cook for him the way i do my family.
And, here's a recipe for Emeril Lagasse's Baby Bam that I make with mine, I put VERY little salt in this, about 3/4 tsp, even though it calls for more. We've even made it and packaged it in plastic storage containers for gifts, and use it to season most everything:
http://www.emerils.com/recipe/6559/
Hang in there<3