Block the intake to the powerhead so that he does not lose another or use a different power head. The Koralia style wrapped in a zippered mesh media bag has worked very well for me with many octos.
If your little guy survives the trauma (stress can be lethal), he should be fine. Arm loss is not a real concern. The injury will heal very quickly. You should start to see a small, thread like growth in a week or so. That back arm(note the thick to thin appearance) and possibly the one next to it that looks blueish both look like they have been poking in holes containing crabs large enough to take a bite.
I've not seen infection where an arm has been severed (eyes are the most common problems) but watch for a swelling at the damage site. Tetracycline is available for fish without a prescription but determining dosage and successfully getting an octopus to ingest it is a lot of experimentation and second guessing.
There are no known, externally obvious features to declare an octopus as female. Here is a link with pictures to help determine if an octopus is a male. Try to observe the third arm on the right (ie clockwise as you orient your eye with the octos). If the animal is a sexually mature male, you should be able to notice that he keeps this arm curled most of the time. The tip of this arm (hectocotylus) is also suckerless and modified to be somewhat funnel shaped (ligula not often easy to detect). Additionally, there will be a channel that runs down the length of this arm that is sometimes detectable.