Are you comforting him each time? Saying "don't worry, Mummy's here?"
If you are you could be accidentally reinforcing his fear. Your responses tell him subconsciously that he was right to be fearful, that he can't manage scary things unless mummy steps in.
You might need to begin working on bravery with him, where he expresses fears about things that are actually not a threat. There is a little routine you do: accept his feeling, normalize it, then encourage gentle exposure.
If he says he's scared of a birthday candle, you might say
"It's not surprising it feels a bit scary, it is a tiny little fire and big fires are dangerous (accept the feeling). Lots of people are a bit worried about fire (normalize). But birthday candle flames are so tiny that they can be blown out with just a big breath and we don't need to be worried about them. You can practice doing a big breath and blowing out a candle using my finger. (Hold up index finger) Ready, steady, BLOW! (Blows finger right over) Wow, that was a big breath! Let's practice on a real birthday candle and see how powerful your breath is!" Etc...
Grandparents? "I understand that Granny and Grandpa might make you feel a bit worried when they arrive because they want cuddles before you feel ready. The thing is, grannies and grandpas love their grandchildren very much, and because they are old they can easily forget what it feels like to be a bit small. I will ask them to show that they love you in a different way. How about we buy a new story book and when they come they can have a cup of tea, and then read your story with you on the sofa?"
You get the idea. You don't dismiss his fears as nothing to worry about, because he IS worried, neither do you give him the impression he can't do the scary thing, or that you need to protect him. You rationalize and then find a bravery step forward.