Brenda,
I understand the stress of a child not eating. Our daughter not only is a picky eater but also a preemie and still at 18 months not even close to 20 pounds. I totally again about making the mealtime a happy experience for them. Let them play/paint their tray with milk if that is what they need at that moment. Now I'm able to limit this painting to the end of the meal so we are making progress in many ways.
Things that we have done to help get more nutrients into her and fat are as follows.
Grate carrots and zucchini into the boiling water for the last 2 minutes while cooking mac-n-cheese. Drain and make as always. Hopefully your son won't even know it's there. I've worked my way up to also being able to add really small pieces of tofu to the mac-n-cheese as well.
Mash bananas or grate carrots and apples into pancakes.
Try putting wheat germ inside the grilled cheese sandwich - we make ours with wheat bread.
You can also try mashing banana or adding wheat germ to the yogurt as well.
Some of the other foods my daughter eats that are good for them and good in fats are sunflower butter- I get it at Trader Joe's. You can add this to the pancakes or as a snack roll a banana in it. I sometimes cut up a banana and put sunflower butter (it tastes like peanut butter but less alergenic) on a fork - a small amount and then pick up a piece of banana. We love this with a big glass of milk.
If you can get your son to eat mashed potatoes you can try hinding veggies in that as well as shredded meats. I've put cooked chicken in the mini chopper til it's eraser size shavings and added that to a lot of different things - even pancakes.
I've added broccoli tops to scrambled eggs as well as cheese.
Beans are really good for them too. I've cooked them and take the skin off - my daughter has texture issues - and mashed them with a little milk and added them to different things she eats. Try adding things like this to the stage 3 foods. If he is willing to eat the stage 3 them maybe you can make some of your own foods and put them in the jar so he thinks he's getting one thing but he is getting another. It might be worth a try.
Also the feeding specialist that we have seen suggests offering one new item of food with 3 items they already know at a meal. Even if they don't try it they see it and that is important too. Maybe in the beginning try offering one new item at lunch every other day and see how it goes. If he doesn't like it on his tray, take it off and don't comment about it just go on with the meal.
We have gotten our daughter to eat more when we all eat together. Eating off our dishes - sometimes it works and sometime it doesn't. Sometimes she wants our fork/spoon and that helps, sometimes it doesn't. Food can also be about exploration so I'm going with the flow right now. She is gaining and the doctor is happy with her progress so we are working with the issues she has now.
Just something to note. My newphew at 1 year only ate yogurt and cheerios - that's it!!! I couldn't believe it but that was all he would he. A few months later it all changed. It's been a lot longer than a few months for us but things are improving and the variety is getting more and more.
Hope this helps. I have a ton more suggestions of different foods if you are interested.
L. M