A.K.
Fear is a very powerful emotion. It is especially powerful when you're two years old. Before that time, kids don't know enough to be scared. As the kid gets older, they learn more fears. Usually, they grow out of them, but not always. Some of us adults have fears that stem from childhood.
Here's one thing I can share with you. I teach parent child swimming and toddler swimming. Kids who are afraid to put their faces in the water become more afraid if the parents cajole, bribe, threaten, whatever. It's not because the child is becoming a rebel, it's because they are already irrationally afraid of putting their faces in the water. The more the parent pushes to do it, the kid feels like - it must be bad if my parents are going through all this to get me to do it. So, the fear increases.
So, all I can recommend is that you stay matter of fact about all his fears. Gently show him that the things he is afraid of are just common things. His monkey costume sounds adorable and a lot of effort was made, but you may just have to let it go.
I'll share another short story, both my kids were in the water as infants, we spent a lot of time at pools and lakes. We used to vacation at the ocean. Each kid spent one vacation freaked out by the ocean. They would go in the pool, they would go on the beach, but would not put a toe in the ocean. It was the vacation they would be 2 going on 3 years old. The next summer was fine, we were back in the ocean.
I don't know if any of this helps, but have a HAPPY HALLOWEEN with or without the monkey costume.