Each time he levels up on lv 16 and over, he will try to evolve. There are a few ways to stop evolution:
1) Give it an Everstone and take it off at Lv.50 and level Treeko up one.
2) Press B while he is evolving but don't after he trys to evolve on lv.51. Grovyle will learn Energy Ball.
These are slow ways of teaching in energy ball, but the fast third option ( this does not get you grovyle ) is to teach him it by TM.
Hope this helps :)