I suspect that a psychologist would say this… people have the need to feel valued, sexual desirability is one way that people feel valued, and the fear that one is not valued might create the need to interpret things that suggest you are valued - i.e. the sexual interest interpretations.
So - someone who doesn’t feel good about their looks might tend to interpret sexual interest in very small and trivial actions. It feels good to think people are interested in you. This is true for everyone, pretty much (as long as its not overwhelming). He might subconsciously also want to create some jealousy - to let you know that he’s a valuable guy.
A person who is feeling powerless - might tend to be paranoid about people who are rich and powerful trying to steal his business ideas.
We all do this to one degree or another…
And one way to help your boyfriend is to express your own sexual interest in him, compliment him, try to convey your your interest in him and reasons - so he can help to understand his positive aspects.
At the same time - don’t feed his delusions - talk to him about his feelings. “How did you feel about that situation” “I wouldn’t have interpreted the situation like that - but it obviously upset you”
Be understanding and try to help him understand his feelings.