Ingestion of substrate is no big deal. As long as he isn't swallowing a stick, your BP will be fine. I am not going to say what i use for substrate... don't really want to make this into a debate but I have been using my substrate for three years with no problems at all.

I also have never had a mite problem due to substrate. Only once have I had a snake with mites and that was when I bought my boa, the next day we notice a little black bug on him... one treatment and it was over. We caught it pretty early. I also have never personally met someone who had a snake get mites from substrate but I have heard of it.

I will only say what I know about substrate I have actually used.

My absolute favorite substrate is cypress mulch. I just do not use it because of the cost. One bag doesn't cover much and it is pretty expensive. For one snake though, I highly recommend it. It keeps humidity well and isn't as messy as coconut husk. And it looks great. I never seen mold but sometimes it can produce that molding wood smell.

Aspen... well, as popular as it is, I do not like it. It is horrible for humidity and it just doesn't look natural, if decor is important to you. It wont harm your snake though and it is a good cheaper alternative.

Coconut fiber (brick version) is okay for holding humidity. It lasts longer then aspen not as long as cypress mulch. In heated areas it tends to dry out quickly. It is super messy. The really nice thing about it is it looks really good and works like cat litter. It clumps up pee and poo.

There is also repti bark, I used it for like a week. I personally HATED it. It stained my snakes red despite me rinsing the bark to get rid of the red pigment. It is also hard and seemed pretty uncomfortable for the snakes. You know when you nap and you end up with impressions on your arm... well that is what happened with my snakes when I used repti bark.