I do not practice in MO, but I may offer some general advice. No one on this site can tell you if your husband's concens are real or imaginary. Only those close to him are going to be able to do that.
Assuming his concerns are real, he has several options: Filing criminal charges; obtaining restraining order(s); or civil suit(s).
To file criminal charges, he should contact the local police department/prosecutor's office when he has proof of his stalker's/stalkers' identity(ies). Potential criminal charges include, but are not limited to, disorderly conduct, stalking, harassment, etc.
To obtain a restraining order, he will have to get info on the process from the local clerk's office or a local, experienced attorney familiar with handling restraining orders.
To sue civilly (i.e., harassment, emotional distress, etc.), he should consult with a local, experienced civil litigation attorney.
If your husband has mental issues, you may want to seek a mental commitment. You would need to speak with a local, experienced attorney who handles such matters for further info.
Good luck to the both of you!