Since he has asked for your help, I second the suggestion of finding the nearest PFLaG chapter and attending. You both can go together or on your own. Very family-friendly and a great place to ask a lot of questions. Be open to your child's questions and honest about your own uncertainty when you don't have the answer. Most important message you can send is simply unconditional love and acceptance. ( I know you know this. Sometimes it is just nice to hear it reinforced.)

I am straight and recently started to work with a transman and then found a friend transwoman. I have only one thing to say, pronouns suck. I try as I know it is hard to be who you are. So hard that people have been killed for it. I wish to give all the support I can. Not hard to support what is hard is to refer to someone who looks womanly him, and vice versa. Pronouns suck. Decided that when referring to another person their name is the proper manner to do this. Pronouns suck.

Start by asking their pronouns, there are people on the gender spectrum who identify as both or neither gender and it isn't uncommon for someone so young to just roll with what makes their loved ones comfortable instead of the other way around. Surprisingly the language is so much more complicated than most people are used to, about a month ago i learned about 15 new pronouns that they never taught me in english because the language evolves to suit the culture. PFLAG is an excellent resource, you're on the right track. Find positive representations in media for them so that they can see themselves represented in the wider world, comics seem to be better at it these days, young adult fiction also tends to do better than most of the stuff for older readers because it tends to be written by and for people in the community.

How about nail polish or make-up. Both can be easily changed or removed. The 2 of you might hit up a thrift store for clothing so he can try some different fashions.

Ask him what he wants and if he wants your help.

We are particularly close and he always wants to do things with me. He even invited me to play D&D with his friends and really wants me to learn Magic the Gathering.
We’d come up with a few ideas (shaping beard, coloring hair) but wanted to see if anyone had any other ideas.

Props to you, mom, for being supportive. I have zero experience in this area, but my guess is there are probably organizations, possibly even local, who can provide guidance.