About darkening hairs...meh.
I used to do this a couple of years ago; at the time, I'd known I was trans for a while, but wasn't out to many people at the time. Back then, I thought drawing on my facial hair was really fun and non-gender binary... did I mention I used to look really androgynous looking back then too? Eh, I dunno. Maybe It's just me who feels this way, but ever since I started T, drawing on facial hair as if it should be part of my regular routine is borderline dysphoria inducing - even to think about. I guess it's because I feel like I'm trying way too hard to fill in the blanks. I mean, facial hair is all based on your genetics; some people hardly develop any at all, even cis males. Others just take a while to fill in. I'm sure he knows if he should be able to grow facial hair or not, or he wouldn't be panicking. I'm sure he's drawing it on because he is unhappy with himself and he's trying to compensate for the facial hair he's been desperately waiting for.
I'd link you to Propia, but they don't appear to have anything in stock at the moment, so...I really can't find anything better at the moment, unless you are willing to teach him a new DIY method for his facial hair.