Hi! I believe I can be of help with this issue.
First, let me say I can imagine how confusing and frustrating this situation is for you. You are clearly a loving and caring parent and your son is all of a sudden coming up with this crazy problem.
And this is actually the key to my answer to you that you need to consider and think about. Your son is 5 years old and 5 year olds DO come up with some crazy fears suddenly. You have no way to learn what triggered it, whether it was something he saw on TV, something someone said in school, or what it was. And whatever it was, it may not have been about plumbing even. But your son made a connection. Because 5 year olds engage in magical thinking. They are not cognitively fully formed.
Therefore, I urge you not to try to convince him with logical arguments that are clear and obviously true to you who are cognitively sophisticated. His brain does not think that way yet. A toy actually is a train or car or ship to him. So plumbing can do strange things in a 5 year old world. So what to do?
Start out by NOT trying to convince him there's nothing to be worried about. But you CAN work with him on feeling that whatever might scare him is not going to harm him. Partly because you're there and partly because he's stronger than he thinks. This has to be done on his level to work:
That you don't have to be in the bathroom while he brushes his teeth to be his protector. That if he sees a monster or whatever it is, that you will be there lickety split and everything will be fine. And you play with him on how he can use the toothbrush to hold off the monster till you arrive!!
Yes, it's okay to play pretend games that help him feel more powerful and participate in being part of the safety system of his life and the house.
Now another aspect. Often when kids have these sudden fears it is a developmental leap waiting to happen. Meaning that he might be being expected to be on his best behavior and most grown up self in kindergarten or day care, etc. And he's not quite there yet. So it's causing stress
and it comes out in unrelated fears.
He has to decompress. At home, he needs to be allowed to be developmentally younger when he comes home from school. If he has some favorite games or toys from 1or more years ago, bring them out to play with him and see if he wants to. Don't push him, but let him be younger. He has a lot of kid development to make up for that he didn't get through because he had to try to cope with keeping the world safe and understandable. So, let him be young. If it means playing cuddly games on your lap, fantastic! If it means wrestling with dad like when he was 3-4 years old, great! As long as dad lets him win every time like when he was 4 years old. At school, he has to contend with lots of expectations. So let him decompress at home.
Does that mean permissiveness? No. He has to follow rules. And consequences. And you don't waiver from them. You must be consistent. But outside of that, let him be young. It's emotional safety and trust he needs.
Second, work with him with some books. These two are excellent for their suggestions. All the books are available easily online.
What to Do When You're Scared and Worried: A Guide for Kids by James Crist. This is a bit too old for your son but look at it anyways and see if you can use it with him as it's SO good.
What To Do When You Dread Your Bed by Dawn Huebner. She's excellent in all her books and your son may relate to this one nicely even though he's young. Use the pipes in place of the bed.
Now these next two are very age appropriate and he might find them very reassuring:
Night Light: A Story for Children Afraid of the Dark by Jack Dutro. Again adapt.
Scaredy Squirrel by Melanie Watt. He might like all her books.
Okay, I wish you the very best!
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