I clean hulls for a living and have done for 15 years. I speak from experience.
Tell your son that the $100 compressor from Home Depot is perfectly safe to use, providing that it is an oil-less compressor. There are no compressors designed specifically to provide breathing air, at least not at the consumer level. Commercial dive companies will use surface supplied air, but we are not talking about small, portable hookahs, we are talking about large units that cost many thousands of dollars and have multiple diver and communication capabilities etc. Not suitable for the hull diver.
The only difference between the compressors found on the shelf at Home Depot and the ones sold in hookah rigs are overall quality of design and some bells and whistles. For instance, many compressors come with reserve tanks, which are generally unsuitable for use in a marine environment and basically unnecessary for this application anyway. Also, the cheaper compressors (like the $100 unit your son is considering) are much less durable than compressors costing only $200.
The most common compressor used in the hull cleaning industry is the Thomas 1020. They cost about $225 or less. They are small, extremely rugged and easy to repair. Can't buy them off-the-shelf (at least I've never found them) but Amazon.com and Toolbarn.com have them all the time.
Regarding breathing hose and regulators; it is critical that your son only use hose rated to provide breathing air. Pneumatic tool-style hoses are unsuitable and dangerous to use. 2nd stage SCUBA regs may or may not work in a low-pressure hookah application. Some do, some don't. To avoid any issues in this regard, I use regs that have been modified for use in a hookah setup.
BTW, here's a link to the other hookah thread previously mentioned:
A bottom cleaners Hooka