COVID-19: Chapter 10 - Mission Achomlished!

Hey folks!
A while back I wrote about a debate I was having in terms of getting my then-11-yr-old son vaxxed. (Cliffs: I was wondering about waiting a couple months to get him the ‘stronger’ dose given to 12-year olds. I took the thread’s advice and got him what was available ASAP).

New questions:
We’re heading to the states for the next 5 weeks or so. He has not yet become eligible for a booster here (he will become so here while we’re abroad in the US) so we haven’t yet. Also potentially key info: he has a US passport.

I want to get him vaxxed again or boosted while we’re in the States. Can I walk him into a pharmacy for that? Should I say he’s never been vaxxed? Should I tell the truth or might that cause problems? Finally, his first shot was Pfizer… am I reading this thread correctly that the smart play would be to get him Moderna now, if we have a choice?