Yes, PLI in case he is blamed for an accident/damage. I'd think carefully about personal accident insurance (if you mean the type that would pay out if he's injured and cannot work) and read the terms of the policy carefully as it could be expensive and have exclusions - while he's still living at home, it might be just better to just have savings instead and manage without an income.
Has he also registered for self assessment? He can claim the cost of insurance, tools and safety equipment off his tax and also travel to work, so he needs to keep records of this.
Does he understand how the money side works? Being on CIS will mean that he might not get huge tax bills (and indeed might get a refund depending on how much he earns after expenses) but he needs to save for tax and he also needs to recognise that if he's not working (eg holidays, sickness, Christmas shutdown or between contracts) he's not earning, so he should put money aside to cover this. He should also start a pension, which is more important when SE as there's no employer contribution. He should expect to put aside 30-50% of his income for tax, pension and a buffer for the times he's not earning, otherwise he'll come unstuck at some point.
But good luck to him. If he's good at the job and reliable (seriously, turning up on time, sober and getting on with the job instead of messing about puts you ahead of many) it could be the start of a decently paid career and he could have the opportunity to move between trades as he'll be able to see all the different jobs and hopefully get the opportunity to try them out.
DP used to work on housebuilding sites as a machine driver and has had licences to drive 3 or 4 different types like diggers, telehandlers, etc and once he got a foot in the door he earned quite decent money, particularly as all training is either on the job or a course that's a few days long. I also know a couple of guys who drive the really big machines - wrecking balls, cranes etc and the good ones with their own rig can be really in demand and more or less name their price.