Tell him to add $100/day to cover the "hidden" expenses of tips and drinks and he will be able to have a good time.
He will have the chance to relax or have fun that he has at an all inclusive but he will see so much more than he ever could at an all inclusive. Multiple ports to see, snorkeling in different areas and more activities on board or relaxing in the sun. You can keep on shore expenses down by doing things on your own or taking in the beaches.
I've done both and in the end the price is almost equal unless you are a big drinker. My wife and are moderate drinkers (2 or 3 mixed drinks a day and wine with dinner). I don't drink too much any more so my onboard bar bill is less. Note: We usually go with a balcony cabin or higher and in a all inclusive we have done an ocean-view with balcony.
The negative is that you do have to watch or budget the "impulse" purchases such as: Spa, shops, photography, etc. These are where you will add to your onboard account. However these are also extras in most all inclusives.
In the end, being on the ocean is more enjoyable than standing on the shore looking at it.