Any other father would have had reservations about making such a purchase/gift, given that he was aware of the kid making "school shooting" threats specifically.
The father is being charged with involuntary manslaughter. Speaking abstractly, know something is dangerous and doing nothing about it entails, at least, civil liability (e.g. a known-dead tree falling on a car or house). In this case, the father knowing his son may have made deadly threats (the father apparently did not believe his son had done so, however) which the son carried out may cross the line into criminal liability. That is why he is being charged. How that would play out is outside of my "pay grade" (I work in tech) and there may be myriad devils in the details.
BTW, the local authorities (police/sheriff? school admins? all of the above? not sure) may not be entirely off the hook, as the FBI did advise that they keep an eye on the kid.