The purchase of a vehicle prior to your grandson passing his test is not a great idea, it may well lead to the temptation to drive it. It will also need to be insured even whilst not in use as it a legal requirement to have a policy of insurance in continual enforcement. This is costing money with little benefit, that and it will need to also be taxed. Then you also have unqualified people giving instruction. The most economical way to move forward is for him to pass his test first, then get the car.
If you have the car now, How is it insured? You could stick your son on that policy. If his parents drive, they could insure as a second vehicle on a family policy to utilise their no claims bonus.

You would probably benefit using www.coveredlearnerdriver.com or www.insurelearnerdriver.co.uk. Once he has passed his test get a black box and do Pass Plus course.