I wonder whether this is a spatial awareness problem?
I remember mum having problems with steps - she would lift her feet up to step up well before she actually reached the steps. I also remember a lady in mums care home who would try and sit down on empty space thinking that the chair was there.
Perhaps he can see you but cant work out where you are in space, so he is touching your arm to reassure himself that you are sat next to him.
My OH has been unable to walk beside me when we're out for some years now. He has to follow close behind me to keep me in view. I turn around every so often to make sure he's still there; could actually do with a rear view mirror. I'm considering some kind of wristbank which I used to use for our toddlers; does anyone know if these exist? For some time recently he can't find me indoors and paces about looking for me. I think @canary is right, it seems to be spatial.
@Libbybookworm Yes, the same thing happens when we're walking. Just can't get him to stay beside me unless I hang on to the handle of the rollator but that's not always possible on the dodgy old pavements round here. I've considered the wriststrap too as he'll wander off in a different direction. Rear-view mirrors needed indeed.
I've lost count of the number of times he's got up close with other females in a shop, assuming that they're me. Doesn't matter the age, height, weight, hair or skin colour, hat, specs etc ... it seems he makes a beeline for the nearest pair of jeggins and padded zip jacket. He's had a few lucky escapes when the female has been about to call for security to come and rescue her. Makes for an interesting shopping trip though.