Thinking about giving this a try

You don't want those seals too tight.  It's not
Good for you.  You can trim them a little at a time until they are comfortable enough to wear for long periods of time.  If you don't want to trim you can put gallon bleach jugs with water in the neck so it will stretch out a bit.

I've heard of that for latex gaskets, but hadn't heard about doing that for neoprene gaskets like the Kokatat Angler suit has for the neck.

I just trim mine, but it's worth a shot if he's a little weary of trimming