If he's on HRT, his clitoris will probably be large and relatively sensitive, and there's a good chance a lot of his sexual pleasure will derive from direct clitoral stimulation (think of it like the tip of your dick). You generally want to keep your tongue flattened and work with broad strokes. Figure out if he prefers a variety of patterns (spelling the alphabet) or a consistent pattern (figure 8s, clitty tickling, ect).

and

Tease your way to the clit and treat it likely a really sensitive penis gland. The lips of the opening are also sensitive so you can treat it a bit like rimming. You can also use a finger or two as part of the fun.

So basically like giving a vagina oral sex. He would have gotten the same answers on a women only / lesbian forum.