Jumping 2 grades seems to be too aggressive, there are a lot of basic techniques in G2 and 3. Age 7 finishing G1 exam is at the right age and level. If I were you, I won't let him skip so many grades.
In fact, good results in Primary or G1,2 are very common. In Jean M. Wong Ballet, they only allow students with result of 100 marks to jump 1 grade.
Examination Grades and Presentation Classes
Minimum Age for Entry
Grade 8 Award
Grade 7
Grade 6
11 years or over
Grade 5
Grade 4
Grade 3
Grade 2
Grade 1
7 years or over
Primary 6 years or over
Pre-Primary (Presentation Class only) 5 years or over