My son is also 5 and kind of like this. He's always working out double-digit problems in his head (sometimes multiplying by saying 30+30+30+30 is 120!), but the other day hubby was trying to do missing addend problems with him (in a different language) and DS was frustrating hubby by not getting/knowing it.
This is apparently a common characteristic of visual-spatial learners, which might be something to look into. My son scored in the 99.9th percentile for visual-spatial skills, so I'm pretty sure this is him.
