So sorry for your situation, but if nothing was on paper all you have is a verbal agreement which he may try to say is not binding.

You may try to ask him to pay you back for the improvements you paid money for but really you enjoyed the benefits of those improvements.

As your grandparents willed it to your father, and not you, I am sorry but you are sort of stuck as he has clearly changed his mind. People tend to do that. If it is not in writing you cannot really do much unless you can prove a binding verbal agreement.

I don't know if it exists in Canada, but in the U.S. there's a theory of property ownership known as
adverse possession by which a person can gain ownership of real property that legally belongs to someone else. Basically, you act as a sort of squatter; you move onto someone's land and treat it like it's your own and - if enough time goes by without a challenege - you can claim ownership.

I don't know if Canada has anything similar, but if so you may have a claim there. However, it's usually not particularly cut-and-dry. Your claim on the land has to be open and notorious; you can't stealthily move onto someone's property. Plus, you have to take actions that make it clear that you're claiming ownership: putting up fences, making improvements, paying taxes, etc.

Not knowing anything about Canada's laws, it's clear that you need to consult with counsel to see if some form of adverse possession exists there. Outside of that context, it's unfortunate that you don't have anything in writing, because under the law I think your father inherits.

And as I'm thinking about this one last thing occurs to me: there's an argument that your grandparents left you a "life estate" in the house. Basically, a life estate would grant you the right to live in the house as long as you wanted, with your father "inheriting" afterwards. However, a life estate usually works in the opposite direction, meaning that what we'd expect to happen is that your grandparents might give your father (their child) a life estate in the house to make sure he always had a place to stay, with the remainder going to you (meaning you'd get the house when your father moved out or passed away).. Here, because you're probably expected to outlive your father, a life estate to you with your father inheriting afterwards really doesn't make sense." Oh well, it was just a thought; a good lawyer might be able to run with it.