Sometimes, excitement over the subject of evolution isn’t excitement over evolution itself but rather excitement towards an evolved form. Think of it like this. Many of us have heroes, people who are more advanced that we are in our respective fields. However, not all of us want to literally be those heroes. Rather, we all have our strengths and weaknesses, and sometimes, we embrace or prefer ourselves as we are. (For example, I may idolize Professor Oak as many researchers my age do, but none of us want to literally be him. I for one would not want to change anything about myself; I simply want to use this skills I already have or will develop on my own to be as great a researcher as he is.)
In much the same way, it’s very possible that your ivysaur doesn’t actually want to evolve. He may simply idolize venusaur (or even a specific venusaur) for their power. It may be worth it to try to communicate with him in order to see if this is the case, but if you still can’t quite understand him, try training him to use similar abilities to a typical venusaur. For example, try working on increasing the power of his Solar Beam (or work on teaching him that move) until it’s just as powerful as that of his evolved form. Teach him new, powerful techniques. Find things he may be able to do as an ivysaur that he wouldn’t as a venusaur (he’s smaller and lighter, for one) and incorporate that into his battle style. It is, in other words, all about helping him grow and fulfill his wish to be as excellent a battler as something most definitely stronger than he is so that, perhaps, he’ll feel as if he’s living up to his idol’s standards.
If, however, his behavior isn’t because he idolizes venusaur, it could simply be that he’s indecisive on the matter, but the extra training will give him time to settle on what he would like to do.
Best of luck to you and your ivysaur, anonymous!
The Pokédex According to Bill 