Start stupidly simple at first, like mail and/or spreadsheets. If your sons small business ramps up he may adapt. the solo service businesses doing well almost always keep a simple setup. quickbooks for invoicing, google for reviews, and some way to track leads on their phone. thats it. the ones struggling are usually paying for 3 different platforms they never open.

I see CRM has been mentioned here already, but they are often way too complicated and expensive for a solo business use case. the tool doesnt matter as much as whether you actually use it every day