Ok, there may be no reason for his fear. Kids have odd fears. Mine was afraid of foxes, who knows why?
The idea is to desensitize him in an happy fun way. Do not force him to be near bugs or to see bugs or to think you will kame him look at bugs. That is way too scary.
Instead, pick books, games, candies, whatever that show very basic bugs. no details or really bugs that look like bugs right now - just cute bugs. Get him to be comfortable with that. Butterflies are a good choice. most kids are not afraid of them. then move to moths, dragonflies. Read him cute stories of them. Make a snack called "ants on a log" with raisens on peanut butter on celery. Don't force or push him into anything, but be relaxed and have fun with fake bugs.
Then move to more detailed bugs - ones that look real. Read books about them, but in a lighthearted way. Show him a picture of huge ant houses in Africa. Laugh and say they look like huge mudpies. Tell him little tiny ants did that. Tell him those ants live so far away from us in Africa. Show him pictures of a caterpillar tunring to a butterfly. Have him guess how many eyes a fly has. Laugh at how it would be if we had so many. Crickets are another fun bug, and they are in the story of Pinnochio - and the cricket is actually a good guy.
then, when he is comfortable with that, a good way to look at bugs is at ones on the other side of a glass window. They can't get in, and you can touch the glass and they can't get you.
Give him time and he will probably get over it. If you force him or belittle him, he may take longer.