As someone who has been through this, at a younger age than your child, and before the trans stuff that's happening now was a thing (and who's adult child has actually gone on to transition) there's a few worrying things on this thread.
First and foremost, do not, under any circumstances, go onto Mermaids for support. It is an awful place that pushes transition, calls parents taking a watchful waiting approach transphobic, and is there to advise how to push your child further down this path.
You have gone into panic mode because you feel your son will end his life if you're not immediatly supportive, it's a narrative that is pushed a lot, and not really representative of the world I have been involved in for the last 20 years.
My personal approach at that age was to pretty much ignore it, I allowed my child to wear whatever, from whatever section, whatever toys and hobbies and friends my child wanted, absolutely fine by me. My child also got upset at being called anything to do with their biological sex, so I just made the language neutral.
There wasn't the pressure/social media etc when my child was growing up, so I understand things will be more tricky now, but do try and keep your son offline as much as possible.
Don't go to the gp etc, you just want to be supportive of clothes, hair, nicknames etc, but without doing anything that would make it difficult for your child to have to 'come out' again as a boy. Kids often go through phases with hair, names, toys, clothes etc, it means nothing.
You can absolutely be supportive and allow your child to express himself without placing more importance on it than there needs to be at this stage.
My child knew I was absolutely supportive, and, although many discussions were had along the way, we didn't do anything about it until my child was an adult, which they said was absolutely the right thing to do and are glad now, although there were some tricky times as a teen.
Stop catastrophising, this may well be a phase, it may not be, but the path will last a lifetime so there's absolutely no hurry to do anything at all. Just let your kid be a kid.
I'll say it one more time though, DO NOT JOIN MERMAIDS, the place is a cesspit.