There's nothing wrong with investigating HRT for a gender-nonconforming young son. But please, leave him alone until he goes into puberty and let him tell you whether he's an effeminate gay man or a transwoman. Fifty years of studies have demonstrated that puberty is the bright line: the majority of these boys settle with their sex and come out as gay, but there's a critical minority who find that their gender dysphoria doesn't fade. (As you point out, transgenderism is very rare.) Don't shame your son or push him in any particular direction. Offer insight therapy (neither gender-critical or gender-affirming) to help him figure it out, in case he's struggling with internalized homophobia or other issues that he hasn't discussed with you. Let him know that whether he turns out to be gay or trans (or who knows? even straight!), it's fine with you. That way he'll remain true to himself and won't feel pressured to conform to your or society's expectations.

Also, I have heard from detransitioned males that HRT is not always reversible in boys. I've heard of boys getting stuck with a micro penis or one that never grows to its full size. Sexual function and libido may be negatively affected, and overall growth may be stunted. Parents should have access to a good endocrinologist to help a boy normalize as male if he determines that he's not trans after all.