First, I wouldn't call this excessive just yet. Without knowing what it is that frightens him, it's hard to say if its a "normal" reaction or not. Is he scared of Phineas' spiky hair? Or that what they're doing is super dangerous and somebody could get hurt? To begin, I would strong encourage deep probing to really understand what it is that scares him, and do so with the thought that there truly is something legitimately scary to him, you just need to see it before you can help him overcome a specific fear.
It's also important to avoid any guilt or shaming for said fear. Even if it's ridiculous, it honestly is a real fear and they need to know you're on their side. Be firm in that he doesn't need to be scared, but not in that he shouldn't be scared.
I recently dealt with a timidity issue with my 7 year old son. He opened our back door to toss something in our blue box but missed it. I told him to go outside, pick it up, and put it in the box. He looked at me with absolute terror and shook his head, because there was "a spider there".
Sparing you any lengthy details, the whole exchange was between 5-10 minutes, I'll just say I probed and found out the root of his fear was that it would bite him and it would hurt. So I went out and stood guard by the spider while he put his container in the recycling. (The troll in me wanted to jump and scare him that the spider was coming, but this was one of those times I knew it was srs bsns)
He was about to run back into the house, but I stopped him, because he was still scared. I had him stand beside me and we looked at the spider for a bit. I blew gently to show him how scared and careful the spider was (it crouched into a ball, and this made him laugh). I encouraged him to blow at it, and he did. I felt it was important for him to see the effect he could have on the spider... give him a better idea of, if it came down to a serious 1v1 fight to the death, what he'd be capable of.
Now, fear about things like bugs, or really any 'physical danger', is easier to deal with. I like to point out that we can (not that we SHOULD) crush most bugs with our pinky finger or baby toe, and even if they bite us, it generally doesn't hurt as much as being pinched hard and isn't as annoying as mosquito bites. Even then, it won't kill us. In general, it's about explaining how we are safe or instead understanding the actual degree of danger.
Mental or abstract fears are a bit trickier, but generally it's the same process. Probe and understand the root of the fear, then explain/demonstrate why the fear is unfounded. If that doesn't work, distraction is the next best thing.
My son had some trouble going to sleep at night for a variety of issues. Recalling my own youth and issues with bed time, I figured the problem was just his imagination. While I could explain something away rationally and he'd agree, as soon as I left, he could come up with something new! So instead, I started to put on some music, very quietly, in his room. Worked wonders. He now can focus on the music, and not get distracted by his random thoughts.
- Probe. Find out the root cause of the fear
- Knowledge is power. Explain/rationalize away the fear.
- Prove it. Generally by demonstrating use of said knowledge.
Last step is most important. Without it, they can be scared new knowledge won't work.