Indeed, he shouldn't be doing consulting, he should be learning everything he needs to be able to do the consulting after hours. If an employer doesn't train you, you should train yourself.

I've been tempted more than a few times to take vacation days and line up another gig for the week that I'm off. Double the pay for doing normal work hours?

meh. you're a startup you want to waste your precious time taking a punk kid to task over consulting? you only waste time with shit like this if it's personal. and in that case you have already lost.