My offer letter states I am entitled to the 2.5% interest in the company after completing 2 years. I've done that and so far just getting the brush off. What can I do? or should do?
You can choose to either let it go and hope they eventually make good on their promise, or you can fight for it.
If you want to fight for it you should really consult a lawyer. They can tell you if you have any grounds to stand on. But that is not something we are going to be able to help you with here probably.
If you choose to fight for it realize you are probably not going to make friends with the company exec and may come to regret fighting for that interest. But only you can make that choice.