Well, I personally feel like trial and error are the best engineering strategies at this age. And it's part of the reason I love Legos. And the best engineering ideas started with flights of fantasy....so I have NO issue with the "out there" crazy creations my kiddos come up with, in fact I love it. But it sounds like maybe he running mama ragged with all the questions and needed support?? (Maybe I am misunderstanding??!).
I have found that teaching my son to use youtube is a great lego resource. Of course this is something that has to be monitored (CLOSELY), but once he learns to navigate it, finding tutorials and help is a breeze.
THere are also several Lego books available for purchase. I am not certain they would teach him the exact strategies you are looking for, but they may definitely help guide him. There are many online at amazon, but we often check them out at our library. I can't recommend just one, there are so many and my son likes to get a different book each time. Many of them are just pictures without explanation, but maybe all the examples will show him what is needed. My son gravitates towards the Lego machine books, he likes creating things that can "do something".
You may also consider a Lego camp. Our area recreation centers often offer these. Many times the focus is on building lego machines, but they may also help guide him.