My rule of thumb is that I make sure that the equipment and (lack of) training is not limiting my kids' progress / skill cap in their chosen endeavors.
My daughter plays viola. I rented one until it looked like she would stick with it. I then upgraded to a modest one. Eventually her teacher said she needed a better viola and better strings and a better bow. So I upgraded again. The way she's going, she'll probably need an upgrade again in a year or two (partly due to size; she is a growing teenager).
She also plays tennis. She played with my racquet for a while, then her Mom bought her a nicer one. Her coach said she needed a better racquet, so I upgraded her to an even nicer one. She has goals of making the high school varsity team, so I've offered to arrange lessons at the local swim and racquet club over the winter to help her towards that.
Her brother is a semi-pro video game player (no really, he is!) as well as a video editor and YouTuber (see sig). The variable ping and low download speeds meant upgrading to the faster Internet and a hardwired connection to the router. I also helped him get his first gaming rig set up and then pieces/parts upgraded several times (video card, power supply, monitor, keyboard, mouse, mousepad, gaming chair).
So to apply this to the OP, I would talk to his current hockey coach and say something like, "Hey, I know my son wants to do the competitive tryouts next spring. Given his current skill level, do you think sending him to $400 camp would materially affect his chances of getting in?" If so, and you could afford it, I would go for it. If he's already a shoo-in, or if he has no real chance of making it with the camp or without, or if you can't afford it, I would say it then becomes a situation where he can either not go or earn his own money and pay to go himself. Since he's young, and if you feel ambiguous about it, you could always match $1-for-$1, or $2-for-$1, or whatever ratio makes sense to you.