Help ID book about grandfather and grandson

Hi everyone,

I had a customer ask me if I could identify a book for her, and it just
didn't ring a bell for me.  Maybe it will for one of you.  I hope so,
because the customer is desperate to buy it for her now adult son in his 40s
(he was born 1961) since it was a favorite of his that he still remembers
and wants.

Here is the description she gave me:

The grandfather is in a wheelchair and the grandson is also sick.  To help
pass the time the grandfather teaches his grandson about map-making.  When
the boy recovers and is going to leave, the grandfather gets out of his
wheelchair able to walk.  The book's setting may be Danish (although this is
not certain) and the reading level is between a picture book and an
intermediate grade book (again not certain).

If anyone can identify the book my customer would be ecstatic!

Many thanks,

Anita