DD has had this since September. It was full 1:1, which was 1.5 hours funded by the council, and the last hour the preschool just 'made up' at their cost. But I have found that now, they are just keeping an extra eye on her in the free play time, and using her 1:1 for the structured time. Will keep any eye on the situation, because I have had to sign the accident book twice last week.
For her, 1:1 meant/means that they ensure her safety in the session, because she escapes & climbs, etc. Also, they encourage her to spend longer doing an activity than she would otherwise (she tends to get her apron on, do one stroke of a paint brush then drop it and walk away).
In the structured time, they are there to help her enter in with the activities, but at the moment that generally just means discreetly wandering after her to make sure she is not in danger, because she can't cope with any structured activities.
During physical activities, they would be by her side, helping her. For example an obstacle course would be dangerous for her both physically and in the fact that she can't wait her turn so would run into the path of other children.
If you think he will already need it, it might be worth asking the prospective nursery to contact the Area Inclusion Officer, and ask for an assessment. It was her report that went to Children's services so that funding could be allocated. DD's is secured until she starts school.