You’ll need to register for PAYE with HMRC as an employer.
Baring in mind the tax free allowance is just under £12k, if he’s just working 2days for you, at £80 a day he’ll be way under that.
I’m not sure you’ll need to be worried about pensions either, as auto enrollment is something like £10k (you can still opt in though if you so choose I think)
Just bare in mind that your son will still be entitled to holidays etc... the things that all employees have rights to.
Its easy enough to sort out and deal with to be honest, but I’d get the advice from either your accountant (if you have one) or as Gomo says, try Acas.