Looks like sunscald to me; common in thin barked trees exposed to sun
on the south or se sides. Sets the tree up for various and sundry
fungal diseases.

Acer platanoides, which is what I think you've got, is going to be a
big tree at maturity 40-50'x40-50' -- apparently much too big for where
you've got it, since someone has already pruned it drastically.

Personally, I'd start looking for specimens that fit the space available,
choose one and plant it to replace your norway maple, taking the maple down
as soon as your other tree becomes more established. It's easier and cheaper
sooner than later, and by choosing a species that fits the site size, exposure
and soil type, you'll have a good chance of having a healthy, long-lived tree
that adds value to your property.

http://www.extension.umn.edu/projects/yardandg
arden/diagnostics/mapleother-s.html

http://www.plantamnesty.org/pruning_tips_content.htm

Kay