Haloperidol is a pretty old anti-psychotic, there are much newer ones with fewer or less severe side-effects. But, if it works, then stay on it. The newer ones do have some of the same side-effects though, such as being very tired, flat affect, etc... . However, typically they do wear off after 1-2 weeks.

I'm not sure exactly which ones are injectable but I'll assume Michah is correct. It could be injected in most large muscles but I think the main place is the butt as it's big, it's a muscle, and not a terrible amount of pain as compared with other possible areas.

Just as a little correction, not everyone gets it injected every 30 days. However, it should last for at least 1-2 weeks or up to 30 days. If he cant take the meds or not on time, then this may be a good route to consider.

As it is puncturing the skin, there's a chance for infection so getting it done at a doctor's office or somewhere like that, the people giving the med would probably clean the area properly and make sure no infections are likely. He may get some tiredness from the meds and the area of injection may also get a bit more tired but these shouldn't be too uncommon. One thing to note, he probably will get tired due to the med, so ideally don't try to take anything to give an energy boost. Very unrelated drugs (in terms of what and where they affect) can interfere with each other. For example, st. john's wort (for mild depression/anxiety) and cyclosporine (an immunosuppressant) can interact, and the result is the immunosuppressant not working as well, so in the cases of organ transplants, the organs get rejected. Also, the usual bit of no alcohol and such with the meds.