There's nothing wrong with your approach. The Phoenix 4 simulator is a reasonable representation of the real thing, but let me suggest a few things to get closer to success on your own.

Trimming is the key; on your simulator, change the trim settings arbitrarily (or better yet, have your son change trims on the TX, blind to you) and then fly the Cub. Trim it as necessary to fly S&L. Do this several times to get a feel of how poor trim affects flight....

Do every check defined in the manual to make sure you have the surfaces moving the appropriate amount and in the right direction as you deflect TX sticks. Make sure the CG is as defined in the manual.

Practice the landings, a lot!! I have the P4 Sim and if you can land the Sim, the real thing is a little easier

Learning by yourself with no one experienced to at least check if the model is flight worthy, can be done. That's how I learned 45 years ago, before ARFies and before simulators. Just a simple 4 channel TX with zero bells and whistles. BUT, I had built2-3 airplanes beforemy trainer that flew successfully for someone else (I built those for a guy to earn spending money in high school) so I at least knew something about set-up. With the confidence of a teen I built my trainer out of cardboard and stuck a 15 up front. The fool thing flew great and I learned quickly. So it can be done and I think you will do it