My thoughts too. What exactly is the problem you're trying to solve here?
If your son is struggling with hours building (for what? Commercial? ATPL?), then what's the reason that he's struggling? Funds? Weather? Airplane availability?
If airplane availability is the (only) reason, then buying a plane might not be the only solution. There are plenty owners/operators who are willing to rent you a plane, "dry" (so not including fuel) for an extended period of time. This is normally not publicly advertised, but if you're serious then you might ask around and get a few helpful responses. Also, buying a share in a syndicated airplane might be an option too, although syndicates typically do not want owners that are only going to do hour building in a short period of time, and then sell the share.
And even then, if you decide to buy an aircraft outright, a C172 might not be the best option. Sure, it's a very rugged aircraft that's very suitable for flight training, but for hour building it's very easy to find aircraft that are significantly more frugal, which translates in more flying hours for less cost. Your best bet would be to find something that's using a diesel/Jet-A engine, such as a DA40 or DR400 Ecoflyer. These aircraft have maybe half or 2/3rd the hourly fuel cost of a C172 and that easily offsets the higher depreciation. And they're usually newer, more comfortable and typically better equipped. Or, at the other end of the spectrum, if you're just going to hour-build, find a C150/152.
Then "starting this business". What business? Are you going into the business of renting aircraft to flight schools? Is your son intending to start its own business in flying people around? I don't know what your plans are, but over the years I have seen several flight business fail because the owners were pilots foremost, not businessmen. The flying business is a highly competitive environment and the businesses that succeed are the ones where the owner treats them as businesses, not just as a way to fund an expensive hobby.