No. An adult, committed Christian joining a Methodist church would not need to be confirmed. Why on earth would they? They would simply be received as members “by transfer.”
Or, you can be like me: After a couple of years of attending the Methodist Church of which I am now a member, I mentioned to the gentleman who was then pastor, that I thought I was ready to join.
The next time new members were being received, I

heard my name being mentioned as among the new members…
I still have:shrug: no idea how it was done. It doesn’t matter at this point; there was clearly a procedure for “wants to join, is a baptized Christian, agrees with John (though usually not Charles) Wesley”. At least, I hope so. Otherwise, I have no idea what I am. Probably still a youthful rebel, wandering from church to church, in search of an identity. (Well, I have to admit that I am no longer, at 60, youthful).
Certainly, it is a very much simpler procedure than what I read about here, on conversion to the Catholic Church.
Yes. I would assume that as the OP said, the kid will have to decide when it comes time for confirmation. The UMC does have something called “associate membership” which is open to people who belong to another denomination and don’t want to give up their membership in that denomination, but by and large this is supposed to be for people who have a “home church” some distance away (college students, for instance).
Edwin
My mother belonged to both the Free Methodist and the United Methodist Churches, under the “associate” provision. But that is a very different thing from being both Catholic & Methodist…
I can only offer the OP my sincere sympathy & my prayers, for a situation which must be painful for him & his son. ( I counsel charity for the mom, despite the fact that it makes me angry when a parent does something this bewildering to a child, for what seems to be little or no reason).
C.S. Lewis was a Catholic he just didn’t know it… The guy believed in purgatory and prayers for the dead, case closed. I dare say C.S. Lewis was way more in line with Catholic teaching than the vast majority of protestant teachings.
I believe in Purgatory too–am I a Catholic? The Methodist theologian Jerry Walls believes in Purgatory, while teaching at the Protestant Asbury Theological Seminary (as do I, for that matter, but only as an adjunct online). Is he a Catholic?
Edwin
I believe in Purgatory as well, Methodist that I am. I know quite a number of Methodists who do.