It sounds like a good plan to me.
Public liability would be recommended as there's potential to cause damage to property or members of the public.
For about £120 your son could get a £2,000,000 public liability cover. If he wanted to include cover for for tools and plant that he owns then you're looking around £200. the policy I'd recommend includes products being worked upon which is excluded by most insurers.
I'm happy to chat if you want a full quotation or some further information.
Hoskin Commercial Solutions
Giving advice to business' of all sizes.