I agree he is probably just maturing but you do know your horse better than anyone.
I might suggest having a GOOD equine chiropractor look at him and pay particular attention to the poll area.
If he was a fractious young man, he could have done anything in the pasture when you weren't looking - lol lol
I had a horse like that once
Along with his sacrum being out, so was the Atlas bone which is in the head/neck area. The chiropractor told me horses do, indeed, get headaches. Some just deal with them quietly while others have a change in behavior - sometimes bad behavior but not always.
Even though you have conferred with a nutritionist, you didn't say what the outcome was. Sometimes food allergies can be developed as time goes on and that will change a horse's personality; although that generally plays out in snotty behavior. I also have one of those - he is oat/corn/soy intolerant.
Once you've elminated these final things, then it's probably back to the vet's conclusion that, at age 5, he is maturing and quieting down some