Marty-
It seems you've got several questions or groups of questions to deal with here:
1. Technical questions about soil type, stability, structural integrity of the wall, etc.
2. Personal relationship questions about whether to trust you son's judgment, expertise, etc., and what will happen to your relationship if you don't.
3. How much risk taking you're comfortable with concerning the boat in view of the uncertainty posed by the landfill, etc.
First of all, you really won't get an accurate and definitive answer from a bunch or amateurs like us looking at a few emails, with or without the photos we requested.
It would take a Soil Engineeer with on site observations and lab tests to give you the best answer, and then it would still be a guess based on the information gathered and some probability estimations. And it would be expensive.
I seems to me at this point that if you want to protect your relationship and strengthen it with your son, you should put the burden of the decision making back on him by asking him what changes would have to be made to improve the stability of the site. Does it need paving to keep the water out? Would metal stakes driven into the deep, firm underlying soil, help the top fill from moving? Do the blocks need to be tied together better?, Etc., etc.
This way you'll put your trust in his judgement, and may well get the corrective work, if any, done for the cost of the materials, and improve the site until you trust it much better yourself.
This approach is not without risks, however, as he could say that the site cannot be made safe.
In that case, I'd suggest you let it rest for awhile, then test it by putting some weight similar to your boat on it for a few good wet months or a season, and then if it doesn't show any movement, declare it safe by testing, and park it there anyway!
Good Luck!
Joe.