It is possible that your boyfriend may be experiencing delusional disorder but only an in-person, psychiatric evaluation could confirm that diagnosis. Two possible signs of the disorder include his belief that words, as “codes”, have a hidden meaning and his paranoia with regard to you cheating. The fact that he had an ex-wife who cheated may make him hypersensitive to the possibility of cheating but his intense focus on your alleged infidelity is unusual.
Delusional disorder is a type of mental illness that involves psychosis. Psychosis is a break with reality. Psychosis involves the inability to distinguish what is real from what is not real. A delusion is the belief that something is real, despite evidence to the contrary.
Individuals with delusional disorders experience “non-bizarre delusions.” An example of a non-bizarre delusion is the belief that someone is being poisoned, followed, conspired against, or admired from a distance. There are six main types of delusions associated with delusional disorders. Jealous is one of them. Individuals experiencing a jealous type of delusion believe that their partner or spouse is cheating in the absence of evidence to support their belief.
If your partner is willing, he should undergo a psychiatric evaluation by a mental health professional. With regard to psychotic disorders, treatment is particularly important because generally, delusions do not go away on their own. They can become worse in frequency and in intensity. Medication could reduce or eliminate his delusions. It may not be easy to convince him to see a mental health professional but it would be in his best interest.
Please take care.