I agree. Have you been working a lot at home? My son is also this way and he is 5. Just set the pencil or whatever on the table and let him decide which hand to use. I do agree as well that this should not be causing any problems at school.
It is perfectly normal for ALL kids to be ambidextrous (using both hands) through 5 years. It shouldn't be something that causes further problems in school, though.
Hope this helps!