"I found it works best at first to just let him stay on your shoulders."
Oh hell no! Maybe someday, but not at this early point in your relationship.
On your shoulders he is in a perfect position to bite you in the face. Don't allow him to climb up on your head either.
You don't want him up at your level for now.
Height = dominance, and you need to be the top dog.
The exception to staying tallest is I found that when my iguana behaved badly (when I first got him - he is perfect now after all these years) that holding him straight up at arm's length settled him down rather quickly.
Make sure you have a good hold so he can't wiggle lose and fall, but up at arm's length, he doesn't have anywhere to go and will soon figure that out. And hopefully calm down.
Not sure why that worked, but it did. Yes, you may get scratched up some, but you chose to own an iguana!
Gotta man up and take the growing pains (and scratches!) to make it through to the reasonably well behaved iguana goal.
Best of luck to you. Your goal is obtainable!