Hi,
My husband taps so much during the day, not himself but on the sofa, on cups if he is drinking, he has tapped on the walls also, even some nights when I take him up to bed he will tap on the bed.
This used to drive me crazy, but some how I have learnt to ignore it.
I also found that if he has something in his hands he is less likely to do it, so when he and I are watching tv I usually give him something, he seems to like the stress ball which is soft to squeeze and doesn't make a noise, sometimes him just holding the tv controller is enough.
I don't know if this is part of having dementia or boredom, my husband is not very mobile now, if its a nice day and he starts tapping I go and sit in the garden with him.